[TargetLowering] Remove workaround for old behavior of getShiftAmountTy. NFC

getShiftAmountTy used to directly return the shift amount type from
the target which could be too small for large illegal types. For
example, X86 always returns i8.

The code here detected this and used i32 instead if it won't fit. This
behavior was added to getShiftAmountTy in D112469 so we no longer need
this workaround.