[llvm] [RISCV][MachineCombiner] Add reassociation optimizations for RVV instructions (PR #88307)

[Min-Yih Hsu via llvm-commits]

mshockwave wrote:

> Does this need rebased over [dc25a72](https://github.com/llvm/llvm-project/commit/dc25a72c95d3df746d28687004901ee2c372a573)

Yes, I had rebased this PR against main since #88306 is now merged.

https://github.com/llvm/llvm-project/pull/88307