[SCEVExpander] Use PoisoningVH for OrigFlags

It's common to delete some instructions after using SCEVExpander,
while it is still live (but will not be used afterwards). In that
case, the AssertingVH may trigger. Replace it with a PoisoningVH
so that we only detect the case where the SCEVExpander actually is
used in a problematic fashion after the instruction removal.

The alternative would be to add clear() calls to more code paths.

Fixes #83404.

Author: nikic

llvm/include/llvm/Transforms/Utils/ScalarEvolutionExpander.h

@@ -85,7 +85,7 @@ class SCEVExpander : public SCEVVisitor<SCEVExpander, Value *> {
 
   /// Original flags of instructions for which they were modified. Used
   /// by SCEVExpanderCleaner to undo changes.
-  DenseMap<AssertingVH<Instruction>, PoisonFlags> OrigFlags;
+  DenseMap<PoisoningVH<Instruction>, PoisonFlags> OrigFlags;
 
   // The induction variables generated.
   SmallVector<WeakVH, 2> InsertedIVs;

llvm/test/Transforms/LoopStrengthReduce/X86/pr83404.ll

@@ -0,0 +1,23 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
+; RUN: opt -S -passes=loop-reduce -mtriple=x86_64-unknown-linux-gnu < %s | FileCheck %s
+
+; Make sure this does not assert.
+define void @test() {
+; CHECK-LABEL: define void @test() {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
+; CHECK:       for.body:
+; CHECK-NEXT:    br label [[FOR_BODY]]
+;
+entry:
+  br label %for.body
+
+for.body:
+  %0 = phi i64 [ %dec, %for.body ], [ 0, %entry ]
+  %h.18 = phi i32 [ 1, %for.body ], [ 0, %entry ]
+  %dec = add i64 %0, 1
+  %rem = urem i64 %0, 5
+  %1 = trunc i64 %rem to i32
+  %tobool3.not = icmp eq i32 %h.18, %1
+  br label %for.body
+}