Index: llvm/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp
+++ llvm/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp
@@ -1164,6 +1164,9 @@
   DL = &F.getParent()->getDataLayout();
   bool Changed = false;
   for (BasicBlock &B : F) {
+    if (!DT->isReachableFromEntry(&B))
+      continue;
+
     for (BasicBlock::iterator I = B.begin(), IE = B.end(); I != IE;)
       if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(I++))
         Changed |= splitGEP(GEP);
Index: llvm/test/Transforms/SeparateConstOffsetFromGEP/crash-in-unreachable-code.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/SeparateConstOffsetFromGEP/crash-in-unreachable-code.ll
@@ -0,0 +1,14 @@
+; RUN: opt -mtriple=amdgcn-amd-amdhsa -separate-const-offset-from-gep %s
+
+@gv = external local_unnamed_addr addrspace(3) global [16 x i8], align 16
+
+; The add referencing itself is illegal, except it's in an unreachable block.
+define weak amdgpu_kernel void @foo() {
+entry:
+  ret void
+
+for.body28.i:                                     ; preds = %for.body28.i
+  %arrayidx3389.i = getelementptr inbounds [16 x i8], [16 x i8] addrspace(3)* @gv, i32 0, i32 %inc38.7.i.1
+  %inc38.7.i.1 = add nuw nsw i32 %inc38.7.i.1, 16
+  br label %for.body28.i
+}