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

[Philip Reames via llvm-commits]

preames wrote:

I dug into whether it was easy to extend this interface to run post-legalize, and ran into more complexity than I expected.  It turns out that we're relying on this running pre-legalize to catch some cases on illegal types.  There are possible mul expansions which could handle these after legalization, but they aren't currently implemented.  

I decided the lift required wasn't really worth it, and have an alternate patch for the same change as a DAGCombine here: https://github.com/llvm/llvm-project/pull/88524

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