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

[Luke Lau via llvm-commits]

================
@@ -1647,6 +1870,9 @@ bool RISCVInstrInfo::hasReassociableSibling(const MachineInstr &Inst,
 
 bool RISCVInstrInfo::isAssociativeAndCommutative(const MachineInstr &Inst,
                                                  bool Invert) const {
+  if (isVectorAssociativeAndCommutative(Inst, Invert))
+    return true;
+
----------------
lukel97 wrote:

Nit, you could also use `RISCV::getRVVMCOpcode` and then just add `RISCV::VADD_VV` etc. to the existing switch below, something like

```suggestion
  unsigned Opc = Inst.getOpcode();
  if (unsigned RVVOpc = RISCV::getRVVMCOpcode(Opc))
    Opc = RVVOpc;
```

Then I don't think we would need the separate vector function

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