[SelectionDAG] try harder to convert funnel shift to rotate

Similar to rL337966 - if the DAGCombiner's rotate matching was 
working as expected, I don't think we'd see any test diffs here.

AArch only goes right, and PPC only goes left. 
x86 has both, so no diffs there.

Differential Revision: https://reviews.llvm.org/D50091


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@339359 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed