[NFC] CodeGen: Handle shift amount type in DAGTypeLegalizer::SplitInteger

This patch reverts change to X86TargetLowering::getScalarShiftAmountTy in
rL318727 and move the logic to DAGTypeLegalizer::SplitInteger.

The reason is that getScalarShiftAmountTy returns a shift amount type that
is suitable for common use cases in CodeGen. DAGTypeLegalizer::SplitInteger
is a rare situation which requires a shift amount type larger than what
getScalarShiftAmountTy. In this case, it is more reasonable to do special
handling of shift amount type in DAGTypeLegalizer::SplitInteger only. If
similar situations arises the logic may be moved to a separate function.

Differential Revision: https://reviews.llvm.org/D40320

llvm-svn: 318890