[X86] Lower _mm[256|512]_cmp[.]_mask intrinsics to native llvm IR

Summary:
Lowering some vector comparision builtins to fcmp IR instructions.
This ignores the signaling behaviour specified in the predicate
argument of said builtins.

Affected AVX512 builtins:

__builtin_ia32_cmpps128_mask
__builtin_ia32_cmpps256_mask
__builtin_ia32_cmpps512_mask
__builtin_ia32_cmppd128_mask
__builtin_ia32_cmppd256_mask
__builtin_ia32_cmppd512_mask

Reviewers: craig.topper, uriel.k, RKSimon, andrew.w.kaylor, spatel, scanon, efriedma

Reviewed By: craig.topper, spatel, efriedma

Differential Revision: https://reviews.llvm.org/D45616

llvm-svn: 335339