[llvm] [RISCV] Add macro fusions for Xiangshan (PR #72362)

[Wang Pengcheng via llvm-commits]

wangpc-pp wrote:

I added some MIR tests, but I think I need more data about performance. cc @dtcxzyw @poemonsense 
And, we may not need to support some fusions in LLVM since the patterns don't match what LLVM generates.

https://github.com/llvm/llvm-project/pull/72362