[TargetLowering] prepareUREMEqFold/prepareSREMEqFold - account for non legal shift types

Ensure we tell getShiftAmountTy that we're working with pre-legalized types to prevent cases where the (legalized) shift type can no longer handle the (non-legalized) type width.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=34366