[DAG] Remove DAGTypeLegalizer::GenWidenVectorTruncStores (PR42046)

[RKSimon]

Summary

Just scalarize trunc stores - GenWidenVectorTruncStores does the same thing but is flawed (PR42046) and unused.

[efriedma]

LGTM

This is by inspection - as was mentioned on the ticket, this code isn't active at all in the tests and I've not been able to get it to fire

In theory, you just need the right kind of node. That said, DAGCombiner specifically avoids bulding such a node, so not sure it ever happens in practice. I think I'm okay with this as-is.

[efriedma]

Err, actually, thinking about it a bit more, we should probably delete GenWidenVectorTruncStores in favor of using scalarizeVectorStore(), since they do basically the same thing.

[RKSimon]

Replace GenWidenVectorTruncStores with scalarizeVectorStore

[efriedma]

LGTM

llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp

DAGTypeLegalizer::GenWidenVectorTruncStores(SmallVectorImpl<SDValue> &StChain,
   unsigned Increment = ValEltVT.getSizeInBits() / 8;
   unsigned NumElts = StVT.getVectorNumElements();
   SDValue EOp = DAG.getNode(ISD::EXTRACT_VECTOR_ELT, dl, ValEltVT, ValOp,
                             DAG.getVectorIdxConstant(0, dl));
   StChain.push_back(
       DAG.getTruncStore(Chain, dl, EOp, BasePtr, ST->getPointerInfo(), StEltVT,
                         ST->getOriginalAlign(), MMOFlags, AAInfo));
   unsigned Offset = Increment;
-  for (unsigned i=1; i < NumElts; ++i, Offset += Increment) {
+  for (unsigned Idx = 1; Idx < NumElts; ++Idx, Offset += Increment) {
     SDValue NewBasePtr =
         DAG.getObjectPtrOffset(dl, BasePtr, TypeSize::Fixed(Offset));
     SDValue EOp = DAG.getNode(ISD::EXTRACT_VECTOR_ELT, dl, ValEltVT, ValOp,
-                              DAG.getVectorIdxConstant(0, dl));
+                              DAG.getVectorIdxConstant(Idx, dl));
     StChain.push_back(DAG.getTruncStore(
         Chain, dl, EOp, NewBasePtr, ST->getPointerInfo().getWithOffset(Offset),
         StEltVT, ST->getOriginalAlign(), MMOFlags, AAInfo));
   }
 }
 
 /// Modifies a vector input (widen or narrows) to a vector of NVT.  The
 /// input vector must have the same element type as NVT.