[llvm] [MacroFusion] Support commutable instructions (PR #82751)

[Wang Pengcheng via llvm-commits]

wangpc-pp wrote:

> > I added a `IsCommutable` to control the generation of checking commutable operands, because we may not need it in all cases. @arsenm WDYT?
> 
> Wouldn't that be implied by the opcode in the first place? The instruction will already have isCommutable set

We can't simply use the `isCommutable` of `Instruction`, because `CheckOpcode` predicate accepts a list of `Instruction`, we don't know if all opcodes are commutable. And it's hard to get the opcodes in TableGen emitter, we will need to iterate over the `Predicates` to see if there is a `CheckOpcode` predicate.
A workaroud can be like:
```
class SingleFusion<string name, string fieldName, string desc,
                   Instruction firstInst, Instruction secondInst,
                   MCInstPredicate firstInstPred = TruePred,
                   MCInstPredicate secondInstPred = TruePred,
                   list<FusionPredicate> prolog = [],
                   list<FusionPredicate> epilog = []>
    : SimpleFusion<name, fieldName, desc,
                   CheckAll<!listconcat(
                              [CheckOpcode<[firstInst]>],
                              [firstInstPred])>,
                   CheckAll<!listconcat(
                              [CheckOpcode<[secondInst]>],
                              [secondInstPred])>,
                   prolog, epilog> {
  let IsCommutable = secondInst.isCommutable;
}

def ShiftNAddFusion
  : SingleFusion<"shift-n-add-fusion", "HasShiftNAddFusion",
                 "Enable SLLI+ADD to be fused to shift left by 1/2/3 and add",
                 SLLI, ADD,
                 CheckAny<[CheckImmOperand<2, 1>,
                           CheckImmOperand<2, 2>,
                           CheckImmOperand<2, 3>]>>;
```

https://github.com/llvm/llvm-project/pull/82751