Revert "[IR] Avoid self-referencing values caused by PHI node removal"

This reverts commit b1b2f0a.

Author: ro-i

llvm/lib/IR/BasicBlock.cpp

@@ -556,14 +556,8 @@ void BasicBlock::removePredecessor(BasicBlock *Pred,
     if (NumPreds == 1)
       continue;
 
-    // Try to replace the PHI node with a constant value, but make sure that
-    // this value isn't using the PHI node. (Except it's a PHI node itself. PHI
-    // nodes are allowed to reference themselves.)
-    if (Value *PhiConstant = Phi.hasConstantValue();
-        PhiConstant &&
-        (!isa<Instruction>(PhiConstant) || isa<PHINode>(PhiConstant) ||
-         llvm::all_of(Phi.users(),
-                      [PhiConstant](User *U) { return U != PhiConstant; }))) {
+    // Try to replace the PHI node with a constant value.
+    if (Value *PhiConstant = Phi.hasConstantValue()) {
       Phi.replaceAllUsesWith(PhiConstant);
       Phi.eraseFromParent();
     }

llvm/test/Transforms/JumpThreading/unreachable-loops.ll

@@ -180,64 +180,4 @@ cleanup2343.loopexit4:                            ; preds = %cleanup1491
   unreachable
 }
 
-; This segfaults due to recursion in %C4. Reason: %L6 is identified to be a
-; "partially redundant load" and is replaced by a PHI node. The PHI node is then
-; simplified to be constant and is removed. This leads to %L6 being replaced by
-; %C4, which makes %C4 invalid since it uses %L6.
-; The test case has been generated by the AMD Fuzzing project and simplified
-; manually and by llvm-reduce.
-
-define i32 @constant_phi_leads_to_self_reference() {
-; CHECK-LABEL: @constant_phi_leads_to_self_reference(
-; CHECK-NEXT:    [[A9:%.*]] = alloca i1, align 1
-; CHECK-NEXT:    br label [[F6:%.*]]
-; CHECK:       T3:
-; CHECK-NEXT:    [[L6:%.*]] = phi i1 [ [[C4:%.*]], [[BB6:%.*]] ]
-; CHECK-NEXT:    br label [[BB5:%.*]]
-; CHECK:       BB5:
-; CHECK-NEXT:    [[L10:%.*]] = load i1, ptr [[A9]], align 1
-; CHECK-NEXT:    br i1 [[L10]], label [[BB6]], label [[F6]]
-; CHECK:       BB6:
-; CHECK-NEXT:    [[LGV3:%.*]] = load i1, ptr null, align 1
-; CHECK-NEXT:    [[C4]] = icmp sle i1 [[L6]], true
-; CHECK-NEXT:    store i1 [[C4]], ptr null, align 1
-; CHECK-NEXT:    br i1 [[L6]], label [[F6]], label [[T3:%.*]]
-; CHECK:       F6:
-; CHECK-NEXT:    ret i32 0
-; CHECK:       F7:
-; CHECK-NEXT:    br label [[BB5]]
-;
-  %A9 = alloca i1, align 1
-  br i1 false, label %BB4, label %F6
-
-BB4:                                              ; preds = %0
-  br i1 false, label %F6, label %F1
-
-F1:                                               ; preds = %BB4
-  br i1 false, label %T4, label %T3
-
-T3:                                               ; preds = %T4, %BB6, %F1
-  %L6 = load i1, ptr null, align 1
-  br label %BB5
-
-BB5:                                              ; preds = %F7, %T3
-  %L10 = load i1, ptr %A9, align 1
-  br i1 %L10, label %BB6, label %F6
-
-BB6:                                              ; preds = %BB5
-  %LGV3 = load i1, ptr null, align 1
-  %C4 = icmp sle i1 %L6, true
-  store i1 %C4, ptr null, align 1
-  br i1 %L6, label %F6, label %T3
-
-T4:                                               ; preds = %F1
-  br label %T3
-
-F6:                                               ; preds = %BB6, %BB5, %BB4, %0
-  ret i32 0
-
-F7:                                               ; No predecessors!
-  br label %BB5
-}
-
 !0 = !{!"branch_weights", i32 2146410443, i32 1073205}