[LegalizeTypes][TargetLowering] Merge getShiftAmountTyForConstant into...

[LegalizeTypes][TargetLowering] Merge getShiftAmountTyForConstant into TargetLowering::getShiftAmountTy.

getShiftAmountTyForConstant is a special helper that changes the
shift amount to i32 if the type chosen by
TargetLowering::getShiftAmountTy can't represent all possible values.
This is needed to satisfy an assert in SelectionDAG::getNode.

It requires additional consideration to know when this helper should be used.
I'm not sure that we are always using it when we should.

This patch merges the getShiftAmountTyForConstant handling into
TargetLowering::getShiftAmountTy so we don't need to think about it
anymore.

Technically this may slightly increase compile times since the majority
of callers of getShiftAmountTy won't need this. Hopefully, this isn't
an issue in practice.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D112469