[llvm] [RISCV] Strength reduce mul by 2^M - 3/5/9 (PR #88993)

[Philip Reames via llvm-commits]

preames wrote:

> LGTM.

JFYI, this needs a significant rebase.  I'm going to wait until https://github.com/llvm/llvm-project/pull/89789 lands, and then revise this to use SHL_ADD directly.

https://github.com/llvm/llvm-project/pull/88993