SelectionDAG: Cleanup and simplify FoldConstantArithmetic

SelectionDAG: Cleanup and simplify FoldConstantArithmetic

This cleans up the FoldConstantArithmetic code by factoring out the case
of two ConstantSDNodes into an own function. This avoids unnecessary
complexity for many callers who already have ConstantSDNode arguments.

This also avoids an intermeidate SmallVector datastructure and a loop
over that datastructure.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237651 91177308-0d34-0410-b5e6-96231b3b80d8