This isn't complete, it's just to the point I got tired of working
on it.

XOP was an AMD ISA extension which Intel never adopted.  It is present
on heavy-equipment CPUs (bulldozer, piledriver, etc.), but no longer
supported on Zen.  It never really attained widespread use, but this
could still be useful.

Huge thanks to Jan Ringoš (@tringi) for testing this during
development; I don't have access to any hardware that supports XOP
so Jan's involvement was absolutely critical.

This is just the portable version; we still need to go through and add
accelerated implementations.

Fixes #678

The vector types have alignment requirements, but these functions are
specifically for storing *unaligned* data.  Some compilers (such as
clang 11) have started to generate bad code for the old versions, but
switching over to void* fixes that.

This also moves the _mm_loadu_epi{8,16,32,64} functions from AVX-512
over to SSE2 (for 128-bit) and AVX (for 256-bit), effectively replacing
the simde_x_mm*_loadu_* functions which are now simply aliases for the
AVX-512 functions.

The only real issue here is that our loadu_si* function take a void*
instead of a __m128i* or __m256i*, making them more permissive.  Code
written against SIMDe will allow you to pass, for example, int8_t* data
to these functions without warning, whereas the _loadu_si* functions
will likely trigger a diagnostic.

The solution for this is for code using SIMDe to call functions like
_mm_loadu_epi8 instead of _mm_loadu_si128, even if they don't want to
use AVX-512.  On SSE2, this will simply become a cast and call to
_mm_loadu_si128 and all is good.  On other architectures we avoid
undefined behavior becous void* has no alignment requirements.

That means the only *real* problem is code which ifdefs SIMDe usage.
In C I would suggest casting to void* instead of __m128i* or __m256i*
when calling _mm_loadu_si128 or _mm_loadu_si256; everything will work
as expected.  In C++, though, that will still generate a warning…
probably the best (well, least bad at least) solution there would be
to define a macro to use instead of _mm_loadu_si128/_mm256_loadu_si256
and use an ifdef to define it differently depending on whether you're
using SIMDe or not.

Fixes #691

This allows us to to only include it from files which really need
it (of which there aren't many; currently only SVML), which is
important since it can pull in a lot of extra headers and dramatically
increase compilation time.

Fixes #693

For value outside of (INT32_MIN, INT32_MAX), as well as NaNs, x86
returns INT32_MIN.

I added a SIMDE_FAST_CONVERSION_RANGE macro (which is defined by
default if SIMDE_FAST_MATH is defined) to opt-out of the more accurate
behavior since it's also substantially slower on many platforms.

Fixes #685

Fixes #682

These are based on some StackOverflow answers linked to in this here:
https://stackoverflow.com/questions/65441496/what-is-the-most-efficient-way-to-do-unsigned-64-bit-comparison-on-sse2

Oops, forgot this for the 0.6.0 release!

Add _mm{,256}_{,mask_,mask2_,maskz_}permutex2var_epi{8,16,32,64} intrinsics to permutex2var.[ch]
Add _mm{,256}_{,mask_,mask2_,maskz_}permutex2var_{pd,ps} intrinsics to permutex2var.[ch]

Add AVX, SSE, NEON, Altivec and Wasm translations for most intrinsics.

* operationalize clang bug 44589
* remove unused bug references

This is basically the same false positive as #657.  As of GCC 10 the
diagnostic is no longer emitted in C mode, but it is still emitted in
C++ mode, so I'm removing the version check from the ld3 code, too.

Fixes #676

_mm_prefetch is supposed to take an int, but when optimization enabled
GCC uses an enum _mm_hint instead.  Without optimization _mm_prefetch
is defined as a macro (as it is on clang) since there is no constant
propagation.

Since we can't rely on all hints being defined on all platforms we
define our own, which are not part of the enum _mm_hint.  So, on GCC
we cast each of the hints to enum _mm_hint to avoid a warning about
an invalid conversion.

IMHO this is technically a bug in the GCC API, but I appreciate the
effort at getting the compiler to validate the input so I'm not going
to complain to the GCC folks as long as this doesn't cause additional
breakage.

Fixes #673

Add the following intrinsics to permutexvar.[ch] :
- _mm_{,mask_,maskz_}permutexvar_epi{8,16}
- _mm256_{,mask_,maskz_}permutexvar_epi{8,16,32,64}
- _mm256_{,mask_,maskz_}permutexvar_{pd,ps}

Add translations to AVX, SSE, NEON, Altivec and WASM

Fixes #670

This should just be slightly easier (less difficult) for compilers
since they may be able to convert this to a gather operation on
some architectures.  I'm not going to hold my breath, though.

Fixes #657

Fixes #656

Remove useless for-loops
Remove redundant to_private/from_private
Remove redundant defined(SIMDE_ARM_NEON_A32V7_NATIVE) when we already
  check for SIMDE_NATURAL_VECTOR_SIZE_LE(128)