[X86] Move the promotion of v16i16->v16i8 for avx512f but not avx512bw from...

[X86] Move the promotion of v16i16->v16i8 for avx512f but not avx512bw from lowering to isel. Change to use vpmovzx instead of vpmovsx.

With avx512f but not avx512bw we need to extend to v16i32 then truncate that to to v16i8. Previously we emitted both nodes during lowering, but I'm trying to switch to using target independent nodes and with that switched the extend+truncate wou

This patch changes the implementation to what will be necessary with that patch which helps minimize test diffs.

llvm-svn: 346552