[llvm] [TableGen] Integrate TableGen-based macro fusion (PR #73115)

[Wang Pengcheng via llvm-commits]

================
@@ -323,6 +324,12 @@ class TargetSubtargetInfo : public MCSubtargetInfo {
   /// helps removing redundant copies generated by register allocator when
   /// handling complex eviction chains.
   virtual bool enableSpillageCopyElimination() const { return false; }
+
+  /// Enable macro fusion for this subtarget.
+  virtual bool enableMacroFusion() const { return false; }
----------------
wangpc-pp wrote:

Oh, I get it. The typical usage is like:
```
    if (ST.enableMacroFusion()) {
      ScheduleDAGMI *DAG = createGenericSchedPostRA(C);
      DAG->addMutation(createMacroFusionDAGMutation(ST.getMacroFusions()));
      return DAG;
    }
```
`enableMacroFusion()` is for knowing if we should enable MacroFusion, `getMacroFusions()` returns the predicates.

Yes, I know we can replace `enableMacroFusion()` with `!getMacroFusions().empty()`, but the implementation of `enableMacroFusion()` avoid the creation of `std::vector`.

Let't take RISCV as an example: https://github.com/llvm/llvm-project/pull/72224/commits/1c92927e7d46c2b3c3f173e35afb2eecd4e9ffbf.
`enableMacroFusion()` is a replacement of `RISCVSubtarget::hasMacroFusion()`:
```
bool hasMacroFusion() const {
    return hasLUIADDIFusion() || hasAUIPCADDIFusion() || hasZExtHFusion() ||
           hasZExtWFusion() || hasShiftedZExtWFusion() || hasLDADDFusion();
}
```


https://github.com/llvm/llvm-project/pull/73115