[GlobalISel] Use getPreferredShiftAmountTy in one more G_UBFX combine

Change CombinerHelper::matchBitfieldExtractFromShrAnd to use
getPreferredShiftAmountTy for the shift-amount-like operands of G_UBFX
just like all the other G_[SU]BFX combines do. This better matches the
AMDGPU legality rules for these instructions.

Differential Revision: https://reviews.llvm.org/D116803