[X86] Fold masking into subvector extract builtins.

[X86] Fold masking into subvector extract builtins.

I'm looking into making the select builtins require avx512f, avx512bw, or avx512vl since masking operations generally require those features.

The extract builtins are funny because the 512-bit versions return a 128 or 256 bit vector with masking even when avx512vl is not supported.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@334330 91177308-0d34-0410-b5e6-96231b3b80d8