[SelectionDAG] Merge FoldConstantVectorArithmetic into FoldConstantArithmetic (PR36544)

This patch merges FoldConstantVectorArithmetic back into FoldConstantArithmetic.

Like FoldConstantVectorArithmetic we now handle vector ops with any operand count, but we currently still only handle binops for scalar types - this can be improved in future patches - in particular some common unary/trinary ops still have poor constant folding.

There's one change in functionality causing test changes - FoldConstantVectorArithmetic bails early if the build/splat vector isn't all constant (with some undefs) elements, but FoldConstantArithmetic doesn't - it instead attempts to fold the scalar nodes and bails if they fail to regenerate a constant/undef result, allowing some additional identity/undef patterns to be handled.

Differential Revision: https://reviews.llvm.org/D113300