[SelectionDAG] try to convert funnel shift directly to rotate if legal

If the DAGCombiner's rotate matching was working as expected, 
I don't think we'd see any test diffs here. 

This sidesteps the issue of custom lowering for rotates raised in PR38243:
https://bugs.llvm.org/show_bug.cgi?id=38243
...by only dealing with legal operations.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@337966 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed