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

[Pranav Kant via llvm-commits]

pranavk wrote:

This seems to be breaking one of our internal software. Backend crashes with:

```
ScalarizeVectorOperand Op #4: t94: ch = masked_store<(store (s64) into %ir.add.ptr, !alias.scope !2182)> t12, t98, t11, undef:i64, t96
```

Relevant lines of backtrace:
```
#8 llvm::DAGTypeLegalizer::run() 
#9 0x000055c40ae936b2 llvm::SelectionDAG::LegalizeTypes() 
#10 0x000055c40ae53808 llvm::SelectionDAGISel::CodeGenAndEmitDAG()
```


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