[llvm] [AArch64] Merge scaled and unscaled narrow zero stores (PR #136705)

[Jon Roelofs via llvm-commits]

================
@@ -1539,10 +1538,12 @@ static bool areCandidatesToMergeOrPair(MachineInstr &FirstMI, MachineInstr &MI,
   if (!PairIsValidLdStrOpc)
     return false;
 
-  // FIXME: We don't support merging narrow stores with mixed scaled/unscaled
-  // offsets.
+  // Narrow stores do not have pair-wise opcodes, so constraint their merging
----------------
jroelofs wrote:

```suggestion
  // Narrow stores do not have pair-wise opcodes, so constrain their merging
```

https://github.com/llvm/llvm-project/pull/136705