[XCore] Simplify code that checks for an aligned base plus a constant.

The code now makes use of ComputeMaskedBits,
SelectionDAG::isBaseWithConstantOffset and TargetLowering::isGAPlusOffset
where appropriate reducing the amount of logic needed in XCoreISelLowering.
No intended functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181125 91177308-0d34-0410-b5e6-96231b3b80d8