[X86] Rearrange some of the code around AVX512 sign/zero extends. NFCI

Move the AVX512 code out of LowerAVXExtend. LowerAVXExtend has two callers but one of them pre-checks for AVX-512 so the code is only live from the other caller. So move the AVX-512 checks up to that caller for symmetry.

Move all of the i1 input type code in Lower_AVX512ZeroExend together.

llvm-svn: 319724