[CodeGen] Fix FoldConstantVectorArithmetic for scalable vectors

For now I have changed FoldConstantVectorArithmetic to return early
if we encounter a scalable vector, since the subsequent code assumes
you can perform lane-wise constant folds. However, in future work we
should be able to extend this to look at splats of a constant value
and fold those if possible. I have also added the same code to
FoldConstantArithmetic, since that deals with vectors too.

The warnings I fixed in this patch were being generated by this
existing test:

  CodeGen/AArch64/sve-int-arith.ll

Differential Revision: https://reviews.llvm.org/D79421

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