[SelectionDAG] Merge constant SDNode arithmetic into foldConstantArithmetic

This is the second patch as part of https://bugs.llvm.org/show_bug.cgi?id=36544

Merging in the ConstantSDNode variant of FoldConstantArithmetic. After this, I will begin merging in FoldConstantVectorArithmetic

I've ensured this patch can build & pass all lit tests in Windows and Linux environments.

Patch by @justice_adams (Justice Adams)

Differential Revision: https://reviews.llvm.org/D74881