[x86] narrow a shuffle that doesn't use or set any high elements

This isn't the final fix for our reduction/horizontal codegen, but it takes care 
of a lot of the problems. After we narrow the shuffle, existing combines for 
insert/extract and binops kick in, and we end up with cheaper 128-bit ops.

The avg and mul reduction tests show an existing shuffle lowering hole for 
AVX2/AVX512. I think in its most minimal form this is:
https://bugs.llvm.org/show_bug.cgi?id=40434
...but we might need multiple fixes to get it right.

Differential Revision: https://reviews.llvm.org/D57156

llvm-svn: 352209