Index: llvm/trunk/lib/CodeGen/RegisterCoalescer.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/RegisterCoalescer.cpp
+++ llvm/trunk/lib/CodeGen/RegisterCoalescer.cpp
@@ -1159,7 +1159,8 @@
   LiveInterval &SrcInt = LIS->getInterval(SrcReg);
   SlotIndex CopyIdx = LIS->getInstructionIndex(*CopyMI);
   VNInfo *ValNo = SrcInt.Query(CopyIdx).valueIn();
-  assert(ValNo && "CopyMI input register not live");
+  if (!ValNo)
+    return false;
   if (ValNo->isPHIDef() || ValNo->isUnused())
     return false;
   MachineInstr *DefMI = LIS->getInstructionFromIndex(ValNo->def);
Index: llvm/trunk/test/CodeGen/X86/coalesce-dead-lanes.mir
===================================================================
--- llvm/trunk/test/CodeGen/X86/coalesce-dead-lanes.mir
+++ llvm/trunk/test/CodeGen/X86/coalesce-dead-lanes.mir
@@ -0,0 +1,19 @@
+# RUN: llc -run-pass simple-register-coalescing -O0 -mtriple x86_64-pc-linux-gnu -o - %s | FileCheck %s
+
+---
+name:            foo
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    undef %18.sub_8bit_hi:gr16_abcd = COPY undef $al
+    %1:gr16_abcd = COPY killed %18
+    dead %8:gr8 = COPY undef %1.sub_8bit_hi
+    $al = COPY undef %8
+...
+
+# This used to hit an assertion:
+#   lib/CodeGen/RegisterCoalescer.cpp:1110: RegisterCoalescer::reMaterializeTrivialDef(...): Assertion `ValNo && "CopyMI input register not live"' failed.
+#
+# CHECK_LABEL: name:            foo
+# CHECK:       bb.0:
+# CHECK-NEXT:    $al = COPY undef %2:gr8