[llvm] [DAG] Combine `store + vselect` to `masked_store` (PR #145176)

[Abhishek Kaushik via llvm-commits]

abhishek-kaushik22 wrote:


> It's not possible to create a scalar conditional store in SelectionDAG. It would need to split the basic block and introduce a branch over the store.

Sorry, you are right.  In that case maybe we need stricter checks before we do this combine, I guess 
```cpp
if (!TLI.isOperationLegalOrCustom(ISD::MSTORE, VT) ||
      !TLI.allowsMisalignedMemoryAccesses(VT, AddrSpace, Alignment))
    return SDValue();
```

is not enough. I assumed that the `isOperationLegalOrCustom` will handle cases with illegal types.

https://github.com/llvm/llvm-project/pull/145176