[clang] [X86] Change target of __builtin_ia32_cmp[p|s][s|d] from avx into sse/sse2 (PR #67410)

[Freddy Ye via cfe-commits]

================
@@ -4742,6 +4742,125 @@ static __inline__ __m128d __DEFAULT_FN_ATTRS _mm_castsi128_pd(__m128i __a) {
   return (__m128d)__a;
 }
 
+/// Compares each of the corresponding double-precision values of two
+///    128-bit vectors of [2 x double], using the operation specified by the
+///    immediate integer operand.
+///
+///    Returns a [2 x double] vector consisting of two doubles corresponding to
+///    the two comparison results: zero if the comparison is false, and all 1's
+///    if the comparison is true.
+///
+/// \headerfile <x86intrin.h>
+///
+/// \code
+/// __m128d _mm_cmp_pd(__m128d a, __m128d b, const int c);
+/// \endcode
+///
+/// This intrinsic corresponds to the <c> VCMPPD </c> instruction.
+///
+/// \param a
+///    A 128-bit vector of [2 x double].
+/// \param b
+///    A 128-bit vector of [2 x double].
+/// \param c
+///    An immediate integer operand, with bits [4:0] specifying which comparison
----------------
FreddyLeaf wrote:

```
$ echo "cmpsd   \$8, %xmm1, %xmm0" | llvm-mc --show-encoding
        cmpsd   $8, %xmm1, %xmm0                # encoding: [0xf2,0x0f,0xc2,0xc1,0x08]
$ echo "cmpsd   \$0, %xmm1, %xmm0" | llvm-mc --show-encoding
        cmpeqsd %xmm1, %xmm0                    # encoding: [0xf2,0x0f,0xc2,0xc1,0x00]
```
encoding won't omit higher bits for cmpsd, but maybe hardware will?

https://github.com/llvm/llvm-project/pull/67410