Index: llvm/test/TableGen/GICombinerEmitter/match-tree.td
===================================================================
--- llvm/test/TableGen/GICombinerEmitter/match-tree.td
+++ llvm/test/TableGen/GICombinerEmitter/match-tree.td
@@ -1,6 +1,11 @@
 // RUN: llvm-tblgen -I %p/../../../include -gen-global-isel-combiner \
 // RUN:     -combiners=MyCombinerHelper -gicombiner-stop-after-build %s \
 // RUN:     -o %t.inc | FileCheck %s
+//
+// This only checks that the code emitter does not crash.
+// RUN: llvm-tblgen -I %p/../../../include -gen-global-isel-combiner \
+// RUN:     -combiners=MyCombinerHelper %s \
+// RUN:     -o %t.inc
 
 include "llvm/Target/Target.td"
 include "llvm/Target/GlobalISel/Combine.td"
Index: llvm/utils/TableGen/GlobalISel/CodeExpansions.h
===================================================================
--- llvm/utils/TableGen/GlobalISel/CodeExpansions.h
+++ llvm/utils/TableGen/GlobalISel/CodeExpansions.h
@@ -25,7 +25,8 @@
 public:
   void declare(StringRef Name, StringRef Expansion) {
     bool Inserted = Expansions.try_emplace(Name, Expansion).second;
-    assert(Inserted && "Declared variable twice");
+    // Duplicates are not inserted. The expansion refers to different operands
+    // but to the same virtual register.
     (void)Inserted;
   }
 
Index: llvm/utils/TableGen/GlobalISel/GIMatchTree.cpp
===================================================================
--- llvm/utils/TableGen/GlobalISel/GIMatchTree.cpp
+++ llvm/utils/TableGen/GlobalISel/GIMatchTree.cpp
@@ -762,13 +762,14 @@
 
 void GIMatchTreeVRegDefPartitioner::generatePartitionSelectorCode(
     raw_ostream &OS, StringRef Indent) const {
-  OS << Indent << "Partition = -1\n"
-     << Indent << "if (MIs.size() <= NewInstrID) MIs.resize(NewInstrID + 1);\n"
+  OS << Indent << "Partition = -1;\n"
+     << Indent << "if (MIs.size() <= " << NewInstrID << ") MIs.resize("
+     << (NewInstrID + 1) << ");\n"
      << Indent << "MIs[" << NewInstrID << "] = nullptr;\n"
-     << Indent << "if (MIs[" << InstrID << "].getOperand(" << OpIdx
-     << ").isReg()))\n"
+     << Indent << "if (MIs[" << InstrID << "]->getOperand(" << OpIdx
+     << ").isReg())\n"
      << Indent << "  MIs[" << NewInstrID << "] = MRI.getVRegDef(MIs[" << InstrID
-     << "].getOperand(" << OpIdx << ").getReg()));\n";
+     << "]->getOperand(" << OpIdx << ").getReg());\n";
 
   for (const auto &Pair : ResultToPartition)
     OS << Indent << "if (MIs[" << NewInstrID << "] "