diff --git a/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp b/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
--- a/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
@@ -4475,7 +4475,8 @@
   Register OpLHSLHS = OpLHSDef->getOperand(1).getReg();
   Register OpLHSRHS = OpLHSDef->getOperand(2).getReg();
 
-  if (isConstantOrConstantSplatVector(*MRI.getVRegDef(OpLHSRHS), MRI)) {
+  if (isConstantOrConstantSplatVector(*MRI.getVRegDef(OpLHSRHS), MRI) &&
+      !isConstantOrConstantSplatVector(*MRI.getVRegDef(OpLHSLHS), MRI)) {
     if (isConstantOrConstantSplatVector(*OpRHSDef, MRI)) {
       // (Opc (Opc X, C1), C2) -> (Opc X, (Opc C1, C2))
       MatchInfo = [=](MachineIRBuilder &B) {
diff --git a/llvm/test/CodeGen/AMDGPU/GlobalISel/postlegalizer-combiner-reassoc.mir b/llvm/test/CodeGen/AMDGPU/GlobalISel/postlegalizer-combiner-reassoc.mir
new file mode 100644
--- /dev/null
+++ b/llvm/test/CodeGen/AMDGPU/GlobalISel/postlegalizer-combiner-reassoc.mir
@@ -0,0 +1,30 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 3
+# RUN: llc -global-isel -march=amdgcn -run-pass=amdgpu-postlegalizer-combiner -verify-machineinstrs -o - %s | FileCheck %s
+
+---
+name: test_reassoc_infinite_loop
+legalized: true
+tracksRegLiveness: true
+body:             |
+  bb.1:
+    liveins: $vgpr0, $vgpr1, $vgpr2, $vgpr3, $vgpr4
+
+    ; CHECK-LABEL: name: test_reassoc_infinite_loop
+    ; CHECK: liveins: $vgpr0, $vgpr1, $vgpr2, $vgpr3, $vgpr4
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: [[COPY:%[0-9]+]]:_(s32) = COPY $vgpr0
+    ; CHECK-NEXT: [[C:%[0-9]+]]:_(s32) = G_CONSTANT i32 2
+    ; CHECK-NEXT: [[ADD:%[0-9]+]]:_(s32) = G_ADD [[COPY]], [[C]]
+    ; CHECK-NEXT: $vgpr0 = COPY [[ADD]](s32)
+    ; CHECK-NEXT: SI_RETURN implicit $vgpr0
+    %0:_(s32) = COPY $vgpr0
+    %7:_(s32) = G_CONSTANT i32 0
+    %13:_(s32) = G_CONSTANT i32 1
+    %46:_(s1) = G_ICMP intpred(eq), %7(s32), %7
+    %34:_(s32) = G_SELECT %46(s1), %13, %7
+    %14:_(s32) = COPY %34(s32)
+    %17:_(s32) = G_ADD %0, %14
+    %16:_(s32) = G_ADD %17, %13
+    $vgpr0 = COPY %16(s32)
+    SI_RETURN implicit $vgpr0
+...