Index: llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
===================================================================
--- llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
+++ llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
@@ -1534,6 +1534,15 @@
   if (!I2)
     return false;
 
+  // Handle a case like this:
+  //
+  // %0:_(s64), %1:_(s64) = G_UNMERGE_VALUES %2:_(<2 x s64>)
+  //
+  // Even though %0 and %1 are produced by the same instruction they are not
+  // the same values.
+  if (I1 == I2)
+    return MOP1.getReg() == MOP2.getReg();
+
   // If we have an instruction which loads or stores, we can't guarantee that
   // it is identical.
   //
Index: llvm/test/CodeGen/AMDGPU/GlobalISel/postlegalizercombiner-select.mir
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/AMDGPU/GlobalISel/postlegalizercombiner-select.mir
@@ -0,0 +1,37 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -march=amdgcn -run-pass=amdgpu-postlegalizer-combiner -verify-machineinstrs %s -o - | FileCheck -check-prefix=GCN %s
+
+---
+name:            select_from_different_results_of_unmerge_values
+tracksRegLiveness: true
+body:             |
+  bb.0:
+
+    ; GCN-LABEL: name: select_from_different_results_of_unmerge_values
+    ; GCN: [[UV:%[0-9]+]]:_(s32), [[UV1:%[0-9]+]]:_(s32) = G_UNMERGE_VALUES undef %2:_(<2 x s32>)
+    ; GCN: [[SELECT:%[0-9]+]]:_(s32) = G_SELECT undef %4:_(s1), [[UV]], [[UV1]]
+    ; GCN: $vgpr0 = COPY [[SELECT]](s32)
+    ; GCN: SI_RETURN_TO_EPILOG $vgpr0
+    %0:_(s32), %1:_(s32) = G_UNMERGE_VALUES undef %2:_(<2 x s32>)
+    %3:_(s32) = G_SELECT undef %4:_(s1), %0:_, %1:_
+    $vgpr0 = COPY %3
+    SI_RETURN_TO_EPILOG $vgpr0
+
+...
+
+---
+name:            select_from_same_results_of_unmerge_values
+tracksRegLiveness: true
+body:             |
+  bb.0:
+
+    ; GCN-LABEL: name: select_from_same_results_of_unmerge_values
+    ; GCN: [[UV:%[0-9]+]]:_(s32), [[UV1:%[0-9]+]]:_(s32) = G_UNMERGE_VALUES undef %2:_(<2 x s32>)
+    ; GCN: $vgpr0 = COPY [[UV]](s32)
+    ; GCN: SI_RETURN_TO_EPILOG $vgpr0
+    %0:_(s32), %1:_(s32) = G_UNMERGE_VALUES undef %2:_(<2 x s32>)
+    %3:_(s32) = G_SELECT undef %4:_(s1), %0:_, %0:_
+    $vgpr0 = COPY %3
+    SI_RETURN_TO_EPILOG $vgpr0
+
+...