[X86] Implement MMX intrinsics with SSE equivalents

[RKSimon]

Bugzilla Link	42320
Version	trunk
OS	Windows NT
CC	@topperc,@efriedma-quic,@jyknight,@RKSimon,@rotateright

Extended Description

Similar to what's been proposed recently for gcc, we should investigate promoting MMX intrinsics to SSE equivalents:

https://gcc.gnu.org/ml/gcc-patches/2019-02/msg00061.html

This probably would be best handled in CGBuiltin.cpp - replacing the MMX builtins with SSE equivalents, although some can probably done in headers as well behind a suitable define.

NOTE: This will cause a high number of subvector insertions/extractions, we might need some mechanism to reduce this even without optimizations.

[efriedma-quic]

NOTE: This will cause a high number of subvector insertions/extractions, we
might need some mechanism to reduce this even without optimizations.

If we switch to widening 64-bit vectors by default, instead of promoting them, the conversions would be free, so it wouldn't really matter. Otherwise, yes, this could get messy; we might need special "fake-MMX" intrinsics.

[jyknight]

Being implemented with:
- https://reviews.llvm.org/D86855:
Convert __m64 intrinsics to unconditionally use SSE2 instead of MMX instructions

• https://reviews.llvm.org/D94213:
  Clang: Remove support for 3DNow!, both intrinsics and builtins.
  - https://reviews.llvm.org/D94252:
  Delete (most) of the MMX builtin functions from Clang.