[X86] Stop implicitly enabling avx512vl when avx512bf16 is enabled.

Previously we were doing this so that the 256 bit selectw builtin could be used in the implementation of the 512->256 bit conversion intrinsic.

After this commit we now use a masked convert builtin that will emit the intrinsic call and the 256-bit select from custom code in CGBuiltin. Then the header only needs to call that one intrinsic.

llvm-svn: 360924