[InstCombine] canonicalize raw IR rotate patterns to funnel shift
The final piece of IR-level analysis to allow this was committed with:
Using the intrinsics should improve transforms based on cost models
like vectorization and inlining.
The backend should be prepared too, so we can now canonicalize more
sequences of shift/logic to the intrinsics and know that the end
result should be equal or better to the original code even if the
target does not have an actual rotate instruction.