[llvm] [RISCV] Enable expansion mul expansion to shNadd x, (slli x, c) (PR #87105)

[Philip Reames via llvm-commits]

preames wrote:

For the record, I can also implement this as the dag combine expansion if anyone wants.  I'd actually implemented that originally before figuring out the generic code could handle this case at all.  

https://github.com/llvm/llvm-project/pull/87105