Index: lib/Transforms/Scalar/SCCP.cpp
===================================================================
--- lib/Transforms/Scalar/SCCP.cpp
+++ lib/Transforms/Scalar/SCCP.cpp
@@ -1933,10 +1933,7 @@
     for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
       if (!Solver.isBlockExecutable(&*BB)) {
         LLVM_DEBUG(dbgs() << "  BasicBlock Dead:" << *BB);
-
         ++NumDeadBlocks;
-        NumInstRemoved +=
-            changeToUnreachable(BB->getFirstNonPHI(), /*UseLLVMTrap=*/false);
 
         MadeChanges = true;
 
@@ -1959,6 +1956,17 @@
       }
     }
 
+    // Change dead blocks to unreachable. We do it after replacing constants in
+    // all executable blocks, because changeToUnreachable may remove PHI nodes
+    // in executable blocks we found values for. The function's entry block is
+    // not part of BlocksToErase, so we have to handle it separately.
+    for (BasicBlock *BB : BlocksToErase)
+      NumInstRemoved +=
+          changeToUnreachable(BB->getFirstNonPHI(), /*UseLLVMTrap=*/false);
+    if (!Solver.isBlockExecutable(&F.front()))
+      NumInstRemoved += changeToUnreachable(F.front().getFirstNonPHI(),
+                                            /*UseLLVMTrap=*/false);
+
     // Now that all instructions in the function are constant folded, erase dead
     // blocks, because we can now use ConstantFoldTerminator to get rid of
     // in-edges.
Index: test/Transforms/SCCP/ipsccp-phi-one-pred-dead.ll
===================================================================
--- /dev/null
+++ test/Transforms/SCCP/ipsccp-phi-one-pred-dead.ll
@@ -0,0 +1,39 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -S -ipsccp | FileCheck %s
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @test() {
+; CHECK-LABEL: @test(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label %Flow5.pre
+; CHECK:       Flow6:
+; CHECK-NEXT:    br label %end2
+; CHECK:       Flow5.pre:
+; CHECK-NEXT:    br label %Flow5
+; CHECK:       Flow5:
+; CHECK-NEXT:    br label %Flow6
+; CHECK:       end2:
+; CHECK-NEXT:    unreachable
+;
+entry:
+  br i1 true, label %Flow5.pre, label %Flow5.pre.unreachable
+
+Flow5.pre.unreachable:
+  br label %Flow5
+
+Flow6:
+  br i1 %0, label %end1, label %end2
+
+Flow5.pre:
+  br label %Flow5
+
+Flow5:
+  %0 = phi i1 [ undef, %Flow5.pre ], [ false, %Flow5.pre.unreachable ]
+  br label %Flow6
+
+end1:
+  unreachable
+
+end2:
+  unreachable
+}