[DAGCombiner][RISCV] Return SDValue(N, 0) instead of SDValue() after 2 calls to CombineTo in visitSTORE.

[craig.topper]

Summary

RISC-V found a case where the CombineTo caused N to be CSEd with
an existing node and then deleted. The top level DAGCombiner loop
was surprised to find a node was deleted, but SDValue() was returned
from the visit function.

We need to return SDValue(N, 0) to tell the top level loop that
a change was made, but the worklist updates were already handled.

Fixes #64772.

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

 //===- DAGCombiner.cpp - Implement a DAG node combiner --------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
if (ST->isUnindexed() && ST->isSimple() &&
         // then we could not allow the scalable store removal because we don't
         // know its final size in the end.
         if (ST->getMemoryVT().isScalableVector() ||
             ST1->getMemoryVT().isScalableVector()) {
           if (ST1->getBasePtr() == Ptr &&
               TypeSize::isKnownLE(ST1->getMemoryVT().getStoreSize(),
                                   ST->getMemoryVT().getStoreSize())) {
             CombineTo(ST1, ST1->getChain());
-            return SDValue();
+            return SDValue(N, 0);
           }
         } else {
           const BaseIndexOffset STBase = BaseIndexOffset::match(ST, DAG);
           const BaseIndexOffset ChainBase = BaseIndexOffset::match(ST1, DAG);
           // If this is a store who's preceding store to a subset of the current
           // location and no one other node is chained to that store we can
           // effectively drop the store. Do not remove stores to undef as they
           // may be used as data sinks.
           if (STBase.contains(DAG, ST->getMemoryVT().getFixedSizeInBits(),
                               ChainBase,
                               ST1->getMemoryVT().getFixedSizeInBits())) {
             CombineTo(ST1, ST1->getChain());
-            return SDValue();
+            return SDValue(N, 0);
           }
         }
       }
     }
   }
 
   // If this is an FP_ROUND or TRUNC followed by a store, fold this into a
   // truncating store.  We can do this even if this is already a truncstore.

llvm/test/CodeGen/RISCV/pr64772.ll

+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 2
+; RUN: llc < %s -mtriple=riscv64 | FileCheck %s
+
+define void @f() {
+; CHECK-LABEL: f:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    sb zero, 0(zero)
+; CHECK-NEXT:    ret
+  %B1 = shl i64 -9223372036854775808, 0
+  %LGV6 = load i8, ptr null, align 1
+  %G3 = getelementptr i32, ptr null, i64 %B1
+  %B5 = ashr i64 -9223372036854775808, 0
+  store i1 false, ptr %G3, align 1
+  store i8 1, ptr null, align 1
+  store i1 false, ptr null, align 1
+  ret void
+}