[SelectionDAGBuilder][NFC] Pass LHSTy to getShiftAmountTy rather than RHSTy

r126518 introduced a a type parameter to the getShiftAmountTy target hook. It 
produces the type of the shift (RHSTy), parameterised by the type of the value 
being shifted (LHSTy). SelectionDAGBuilder::visitShift passed RHSTy rather 
than LHSTy and this patch corrects this. The change is a no-op because in LLVM 
IR the LHS and RHS types for a shift must be equal anyway.

llvm-svn: 343955