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

[Luke Lau via llvm-commits]

https://github.com/lukel97 approved this pull request.

Other than that `definesRegister` comment, looks correct to me. Just some questions about structuring, not sure if they're possible though so feel free to ignore

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