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

[Craig Topper via llvm-commits]

topperc 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
> > ```
> > 
> > 
> >     
> >       
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> > 
> >       
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> >     
> >   
> > Relevant lines of backtrace:
> > ```
> > #8 llvm::DAGTypeLegalizer::run() 
> > #9 0x000055c40ae936b2 llvm::SelectionDAG::LegalizeTypes() 
> > #10 0x000055c40ae53808 llvm::SelectionDAGISel::CodeGenAndEmitDAG()
> > ```
> 
> Is the error `"Do not know how to scalarize this operator's operand!"`? I see there is no case in `ScalarizeVectorOperand` to scalarize `ISD::MSTORE`, but it shouldn't be too hard to add one.

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.

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