[SelectionDAGBuilder] Remove LegalTypes=false from a call to getShiftAmountConstant.

getShiftAmountTy will return MVT::i32 if the shift amount
coming from the target's getScalarShiftAmountTy can't reprsent
all possible values. That should eliminate the need to use the
pointer type which is what we do when LegalTypes is false.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D120165