[X86] Prefer blendi over movss/sd when avx512 is enabled unless optimizing for size.

AVX512 doesn't have an immediate controlled blend instruction. But blend throughput is still better than movss/sd on SKX.

This commit changes AVX512 to use the AVX blend instructions instead of MOVSS/MOVSD. This constrains the register allocation since it won't be able to use XMM16-31, but hopefully the increased throughput and reduced port 5 pressure makes up for that.

llvm-svn: 337083