[llvm] [SelectionDAG] Reduce code duplication between getStore, getTruncStore, and getIndexedStore. (PR #137435)

[Sergei Barannikov via llvm-commits]

s-barannikov wrote:

Mutating a node isn't nice. This appears to work:
```C++
      auto *ST = cast<StoreSDNode>(N);
      SDValue NewST = DAG.getStore(ST->getChain(), SDLoc(N), Const64,
                                   ST->getBasePtr(), ST->getOffset(), MemVT,
                                   ST->getMemOperand(), ST->getAddressingMode(),
                                   /*IsTruncating=*/true);
      return ST->isUnindexed()
                 ? DCI.CombineTo(N, NewST, /*AddTo=*/false)
                 : DCI.CombineTo(N, NewST, NewST.getValue(1), /*AddTo=*/false);
```


https://github.com/llvm/llvm-project/pull/137435