[X86] Make _mm_mask_cvtps_ph, _mm_maskz_cvtps_ph, _mm256_mask_cvtps_ph, and...

[X86] Make _mm_mask_cvtps_ph, _mm_maskz_cvtps_ph, _mm256_mask_cvtps_ph, and _mm256_maskz_cvtps_ph aliases for their corresponding cvt_roundps_ph intrinsic.

These intrinsics should always take an immediate for the rounding mode.
The base instruction comes from before EVEX embdedded rounding. The
user should always provide the immediate rather than us assuming
CUR_DIRECTION.

Make the 512-bit versions also explicit aliases instead of copy
pasting the code.

llvm-svn: 363961