[InstCombine] Handle undef when pruning unreachable code

If the branch condition is undef, then behavior is undefined and
neither of the successors are live.

This is to ensure that optimization quality does not decrease
when a constant gets replaced with undef/poison in this context.

Author: nikic

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -3903,15 +3903,21 @@ static bool prepareICWorklistFromFunction(Function &F, const DataLayout &DL,
     // Recursively visit successors.  If this is a branch or switch on a
     // constant, only visit the reachable successor.
     Instruction *TI = BB->getTerminator();
-    if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
-      if (BI->isConditional() && isa<ConstantInt>(BI->getCondition())) {
-        bool CondVal = cast<ConstantInt>(BI->getCondition())->getZExtValue();
+    if (BranchInst *BI = dyn_cast<BranchInst>(TI); BI && BI->isConditional()) {
+      if (isa<UndefValue>(BI->getCondition()))
+        // Branch on undef is UB.
+        continue;
+      if (auto *Cond = dyn_cast<ConstantInt>(BI->getCondition())) {
+        bool CondVal = Cond->getZExtValue();
         BasicBlock *ReachableBB = BI->getSuccessor(!CondVal);
         Worklist.push_back(ReachableBB);
         continue;
       }
     } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
-      if (ConstantInt *Cond = dyn_cast<ConstantInt>(SI->getCondition())) {
+      if (isa<UndefValue>(SI->getCondition()))
+        // Switch on undef is UB.
+        continue;
+      if (auto *Cond = dyn_cast<ConstantInt>(SI->getCondition())) {
         Worklist.push_back(SI->findCaseValue(Cond)->getCaseSuccessor());
         continue;
       }

llvm/test/Transforms/InstCombine/unreachable-code.ll

@@ -52,10 +52,8 @@ define void @br_undef(i1 %x) {
 ; CHECK-SAME: (i1 [[X:%.*]]) {
 ; CHECK-NEXT:    br i1 undef, label [[IF:%.*]], label [[ELSE:%.*]]
 ; CHECK:       if:
-; CHECK-NEXT:    call void @dummy()
 ; CHECK-NEXT:    ret void
 ; CHECK:       else:
-; CHECK-NEXT:    call void @dummy()
 ; CHECK-NEXT:    ret void
 ;
   %c = xor i1 %x, undef
@@ -129,10 +127,8 @@ define void @switch_undef(i32 %x) {
 ; CHECK-NEXT:    i32 0, label [[CASE0:%.*]]
 ; CHECK-NEXT:    ]
 ; CHECK:       case0:
-; CHECK-NEXT:    call void @dummy()
 ; CHECK-NEXT:    ret void
 ; CHECK:       default:
-; CHECK-NEXT:    call void @dummy()
 ; CHECK-NEXT:    ret void
 ;
   %v = xor i32 %x, undef