[DAGCombiner] narrow vector binops when extraction is cheap

Narrowing vector binops came up in the demanded bits discussion in D52912.

I don't think we're going to be able to do this transform in IR as a canonicalization 
because of the risk of creating unsupported widths for vector ops, but we already have 
a DAG TLI hook to allow what I was hoping for: isExtractSubvectorCheap(). This is 
currently enabled for x86, ARM, and AArch64 (although only x86 has existing regression 
test diffs).

This is artificially limited to not look through bitcasts because there are so many 
test diffs already, but that's marked with a TODO and is a small follow-up.

Differential Revision: https://reviews.llvm.org/D53784

llvm-svn: 345602