[X86] Restore the pavg intrinsics.

The pattern we replaced these with may be too hard to match as demonstrated by
PR41496 and PR41316.

This patch restores the intrinsics and then we can start focusing
on the optimizing the intrinsics.

I've mostly reverted the original patch that removed them. Though I modified
the avx512 intrinsics to not have masking built in.

Differential Revision: https://reviews.llvm.org/D60674

llvm-svn: 358427