[InstCombine] Insert instructions before adding them to worklist

Summary:
This patch adds instructions to the InstCombine worklist after they are properly inserted. This way we don't get `<badref>`s printed when logging added instructions.
It also adds a check in `Worklist::Add` that ensures that all added instructions have parents.

Simple test case that illustrates the difference when run with `--debug-only=instcombine`:

```
define i32 @test35(i32 %a, i32 %b) {
  %1 = or i32 %a, 1135
  %2 = or i32 %1, %b
  ret i32 %2
}
```

Before this patch:
```
INSTCOMBINE ITERATION #1 on test35
IC: ADDING: 3 instrs to worklist
IC: Visiting:   %1 = or i32 %a, 1135
IC: Visiting:   %2 = or i32 %1, %b
IC: ADD:   %2 = or i32 %a, %b
IC: Old =   %3 = or i32 %1, %b
    New =   <badref> = or i32 %2, 1135
IC: ADD:   <badref> = or i32 %2, 1135
...
```

With this patch:
```
INSTCOMBINE ITERATION #1 on test35
IC: ADDING: 3 instrs to worklist
IC: Visiting:   %1 = or i32 %a, 1135
IC: Visiting:   %2 = or i32 %1, %b
IC: ADD:   %2 = or i32 %a, %b
IC: Old =   %3 = or i32 %1, %b
    New =   <badref> = or i32 %2, 1135
IC: ADD:   %3 = or i32 %2, 1135
...
```

Reviewers: fhahn, davide, spatel, foad, grosser, nikic

Reviewed By: nikic

Subscribers: nikic, lebedev.ri, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71093

2 files changed, 8 insertions, 6 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index e42cd75..e16919e 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -1189,7 +1189,9 @@ Instruction *InstCombiner::visitZExt(ZExtInst &CI) {
       // zext (or icmp, icmp) -> or (zext icmp), (zext icmp)
       Value *LCast = Builder.CreateZExt(LHS, CI.getType(), LHS->getName());
       Value *RCast = Builder.CreateZExt(RHS, CI.getType(), RHS->getName());
-      BinaryOperator *Or = BinaryOperator::Create(Instruction::Or, LCast, RCast);
+      Value *Or = Builder.CreateOr(LCast, RCast, CI.getName());
+      if (auto *OrInst = dyn_cast<Instruction>(Or))
+        Builder.SetInsertPoint(OrInst);
 
       // Perform the elimination.
       if (auto *LZExt = dyn_cast<ZExtInst>(LCast))
@@ -1197,7 +1199,7 @@ Instruction *InstCombiner::visitZExt(ZExtInst &CI) {
       if (auto *RZExt = dyn_cast<ZExtInst>(RCast))
         transformZExtICmp(RHS, *RZExt);
 
-      return Or;
+      return replaceInstUsesWith(CI, Or);
     }
   }
 
diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index 0c00d652..c894a06 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -3361,10 +3361,6 @@ bool InstCombiner::run() {
         // Move the name to the new instruction first.
         Result->takeName(I);
 
-        // Push the new instruction and any users onto the worklist.
-        Worklist.AddUsersToWorkList(*Result);
-        Worklist.Add(Result);
-
         // Insert the new instruction into the basic block...
         BasicBlock *InstParent = I->getParent();
         BasicBlock::iterator InsertPos = I->getIterator();
@@ -3376,6 +3372,10 @@ bool InstCombiner::run() {
 
         InstParent->getInstList().insert(InsertPos, Result);
 
+        // Push the new instruction and any users onto the worklist.
+        Worklist.AddUsersToWorkList(*Result);
+        Worklist.Add(Result);
+
         eraseInstFromFunction(*I);
       } else {
         LLVM_DEBUG(dbgs() << "IC: Mod = " << OrigI << '\n'