[llvm] [DAGCombine] Support (shl %x, constant) in foldPartialReduceMLAMulOp. (PR #160663)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

> Are there any shifts in vplan that we can allow in place of a mul? If so, that would of course be a follow-up PR.

Yep, but first we need to support constant operands at all: https://github.com/llvm/llvm-project/pull/161092

https://github.com/llvm/llvm-project/pull/160663