diff --git a/llvm/lib/CodeGen/GlobalISel/GISelChangeObserver.cpp b/llvm/lib/CodeGen/GlobalISel/GISelChangeObserver.cpp
--- a/llvm/lib/CodeGen/GlobalISel/GISelChangeObserver.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/GISelChangeObserver.cpp
@@ -26,6 +26,7 @@
 void GISelChangeObserver::finishedChangingAllUsesOfReg() {
   for (auto *ChangedMI : ChangingAllUsesOfReg)
     changedInstr(*ChangedMI);
+  ChangingAllUsesOfReg.clear();
 }
 
 RAIIDelegateInstaller::RAIIDelegateInstaller(MachineFunction &MF,
diff --git a/llvm/lib/Target/AArch64/AArch64PreLegalizerCombiner.cpp b/llvm/lib/Target/AArch64/AArch64PreLegalizerCombiner.cpp
--- a/llvm/lib/Target/AArch64/AArch64PreLegalizerCombiner.cpp
+++ b/llvm/lib/Target/AArch64/AArch64PreLegalizerCombiner.cpp
@@ -43,6 +43,8 @@
   switch (MI.getOpcode()) {
   default:
     return false;
+  case TargetOpcode::COPY:
+    return Helper.tryCombineCopy(MI);
   case TargetOpcode::G_LOAD:
   case TargetOpcode::G_SEXTLOAD:
   case TargetOpcode::G_ZEXTLOAD:
diff --git a/llvm/test/CodeGen/AArch64/GlobalISel/observer-change-crash.mir b/llvm/test/CodeGen/AArch64/GlobalISel/observer-change-crash.mir
new file mode 100644
--- /dev/null
+++ b/llvm/test/CodeGen/AArch64/GlobalISel/observer-change-crash.mir
@@ -0,0 +1,32 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple aarch64 -run-pass=aarch64-prelegalizer-combiner %s -o - | FileCheck %s
+--- |
+  target datalayout = "e-m:o-i64:64-i128:128-n32:64-S128"
+  target triple = "arm64-apple-ios5.0.0"
+
+  define void @test() {
+    ret void
+  }
+
+...
+---
+name:            test
+alignment:       2
+tracksRegLiveness: true
+registers:
+  - { id: 0, class: _ }
+  - { id: 1, class: _ }
+  - { id: 2, class: _ }
+frameInfo:
+  maxCallFrameSize: 0
+body:             |
+  bb.0:
+    ; CHECK-LABEL: name: test
+    ; CHECK: [[DEF:%[0-9]+]]:_(p0) = G_IMPLICIT_DEF
+    ; CHECK: $x0 = COPY [[DEF]](p0)
+    %0:_(p0) = G_IMPLICIT_DEF
+    %1:_(p0) = COPY %0(p0)
+    %2:_(p0) = COPY %1(p0)
+    $x0 = COPY %2(p0)
+
+...