[DAGCombiner] try to move bitcast after extract_subvector

I noticed that we were failing to narrow an x86 ymm math op in a case similar
to the 'madd' test diff. That is because a bitcast is sitting between the math
and the extract subvector and thwarting our pattern matching for narrowing:

       t56: v8i32 = add t59, t58
      t68: v4i64 = bitcast t56
    t73: v2i64 = extract_subvector t68, Constant:i64<2>
  t96: v4i32 = bitcast t73

There are a few wins and neutral diffs in the other tests.

Differential Revision: https://reviews.llvm.org/D61806

llvm-svn: 360541