[AArch64][SVE] Add patterns for integer mla/mls.

We probably want to introduce pseudo-instructions at some point, like
we have for binary operations, but this seems okay for now.

One thing I'm not sure about is whether we should be doing this as a
DAGCombine instead of directly pattern-matching it. I don't see any big
downside to doing it this way, though.

Differential Revision: https://reviews.llvm.org/D85681