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