Index: llvm/lib/Transforms/Utils/Local.cpp
===================================================================
--- llvm/lib/Transforms/Utils/Local.cpp
+++ llvm/lib/Transforms/Utils/Local.cpp
@@ -2604,6 +2604,13 @@
   combineMetadata(ReplInst, I, KnownIDs, false);
 }
 
+static bool exceptedFromValueReplacements(const User *U) {
+  if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(U))
+    if (II->getIntrinsicID() == Intrinsic::is_constant)
+      return true;
+  return false;
+}
+
 template <typename RootType, typename DominatesFn>
 static unsigned replaceDominatedUsesWith(Value *From, Value *To,
                                          const RootType &Root,
@@ -2614,7 +2621,7 @@
   for (Value::use_iterator UI = From->use_begin(), UE = From->use_end();
        UI != UE;) {
     Use &U = *UI++;
-    if (!Dominates(Root, U))
+    if (!Dominates(Root, U) || exceptedFromValueReplacements(U.getUser()))
       continue;
     U.set(To);
     LLVM_DEBUG(dbgs() << "Replace dominated use of '" << From->getName()
@@ -2633,7 +2640,7 @@
        UI != UE;) {
     Use &U = *UI++;
     auto *I = cast<Instruction>(U.getUser());
-    if (I->getParent() == BB)
+    if (I->getParent() == BB || exceptedFromValueReplacements(I))
       continue;
     U.set(To);
     ++Count;
Index: llvm/test/Transforms/GVN/is-constant.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/GVN/is-constant.ll
@@ -0,0 +1,30 @@
+; RUN: opt -gvn -S < %s | FileCheck %s
+;
+; Test that @llvm.is.constant() does not get its argument replaced with a
+; (runtime) constant.
+
+; CHECK-LABEL: define i32 @fun
+; CHECK: %2 = tail call i1 @llvm.is.constant.i32(i32 %shl)
+
+declare i1 @llvm.is.constant.i32(i32)
+
+@a = global i32 0, align 4
+@b = global i32 0, align 4
+
+define i32 @fun() {
+entry:
+  %0 = load i32, i32* @a
+  %shl = shl i32 %0, 12
+  store i32 %shl, i32* @b
+  %1 = icmp eq i32 %shl, 0
+  br i1 %1, label %land.rhs, label %land.end
+
+land.rhs:                                         ; preds = %entry
+  %2 = tail call i1 @llvm.is.constant.i32(i32 %shl)
+  %phi.cast = zext i1 %2 to i32
+  br label %land.end
+
+land.end:                                         ; preds = %land.rhs, %entry
+  %3 = phi i32 [ 0, %entry ], [ %phi.cast, %land.rhs ]
+  ret i32 %3
+}