[X86] Add ktest intrinsics to match gcc and icc.

These aren't documented in the Intel Intrinsics Guide, but are supported by gcc and icc.

Includes these intrinsics:
_ktestc_mask8_u8, _ktestz_mask8_u8, _ktest_mask8_u8
_ktestc_mask16_u8, _ktestz_mask16_u8, _ktest_mask16_u8
_ktestc_mask32_u8, _ktestz_mask32_u8, _ktest_mask32_u8
_ktestc_mask64_u8, _ktestz_mask64_u8, _ktest_mask64_u8

llvm-svn: 341265