[X86] Add k-mask conversion and load/store instrinsics to match gcc and icc.

This adds:
_cvtmask8_u32, _cvtmask16_u32, _cvtmask32_u32, _cvtmask64_u64
_cvtu32_mask8, _cvtu32_mask16, _cvtu32_mask32, _cvtu64_mask64
_load_mask8, _load_mask16, _load_mask32, _load_mask64
_store_mask8, _store_mask16, _store_mask32, _store_mask64

These are currently missing from the Intel Intrinsics Guide webpage.

llvm-svn: 341251