1. Insert custom lowering hooks for ISD::ROTR and ISD::ROTL.

2. Help DAGCombiner recognize zero/sign/any-extended versions of ROTR and ROTL
patterns. This was motivated by the X86/rotate.ll testcase, which should now
generate code for other platforms (and soon-to-come platforms.) Rewrote code
slightly to make it easier to read.

llvm-svn: 35605