[x86] lower shuffle of extracts to AVX2 vperm instructions

I was trying to prevent shuffle regressions while matching more horizontal ops 
and ended up here:
  shuf (extract X, 0), (extract X, 4), Mask --> extract (shuf X, undef, Mask'), 0

The affected tests were added for:
https://bugs.llvm.org/show_bug.cgi?id=34380

This patch won't change the examples in the bug report itself, but we should be 
able to extend this to catch more types.

Differential Revision: https://reviews.llvm.org/D56756

llvm-svn: 351346