[SelectionDag] Updated FoldConstantArithmetic method signature in preparation for...

[SelectionDag] Updated FoldConstantArithmetic method signature in preparation for merge with FoldConstantVectorArithmetic

Updated FoldConstantArithmetic method signature to match that of
FoldConstantVectorArithmetic in preparation for merging the two
functions together

https://bugs.llvm.org/show_bug.cgi?id=36544

This is the first step in combining the various
FoldConstantVectorArithmetic and FoldConstantVectorArithmetic
functions into one FoldConstantArithmetic function.

Differential Revision: https://reviews.llvm.org/D72870