[SelectionDAGBuilder] Simplify visitShift. NFC

This code was detecting whether the value returned by getShiftAmountTy
can represent all shift amounts. If not, it would use MVT::i32 as a
placeholder. getShiftAmountTy was updated last year to return i32
if the type returned by the target couldn't represent all values.

This means the MVT::i32 case here is dead and can the logic can
be simplified.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D120164