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
@@ -722,6 +722,18 @@
   // anyway for most targets.
   if (!isPowerOf2_32(NewSizeBits))
     return false;
+
+  const MachineMemOperand &MMO = LoadDef->getMMO();
+  LegalityQuery::MemDesc MMDesc;
+  MMDesc.MemoryTy = LLT::scalar(NewSizeBits);
+  MMDesc.AlignInBits = MMO.getAlign().value() * 8;
+  MMDesc.Ordering = MMO.getSuccessOrdering();
+  if (!isLegalOrBeforeLegalizer({TargetOpcode::G_SEXTLOAD,
+                                 {MRI.getType(LoadDef->getDstReg()),
+                                  MRI.getType(LoadDef->getPointerReg())},
+                                 {MMDesc}}))
+    return false;
+
   MatchInfo = std::make_tuple(LoadDef->getDstReg(), NewSizeBits);
   return true;
 }
diff --git a/llvm/test/CodeGen/AMDGPU/GlobalISel/postlegalizer-combiner-sextload-from-sextinreg.mir b/llvm/test/CodeGen/AMDGPU/GlobalISel/postlegalizer-combiner-sextload-from-sextinreg.mir
new file mode 100644
--- /dev/null
+++ b/llvm/test/CodeGen/AMDGPU/GlobalISel/postlegalizer-combiner-sextload-from-sextinreg.mir
@@ -0,0 +1,22 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -global-isel -mtriple=amdgcn-amd-amdhsa -mcpu=gfx900 -run-pass=amdgpu-postlegalizer-combiner -verify-machineinstrs -o - %s | FileCheck %s
+
+# Post-legalizer should not generate illegal extending loads
+---
+name: sextload_from_inreg
+tracksRegLiveness: true
+legalized: true
+body: |
+  bb.0:
+    liveins: $vgpr0_vgpr1
+    ; CHECK-LABEL: name: sextload_from_inreg
+    ; CHECK: liveins: $vgpr0_vgpr1
+    ; CHECK: [[COPY:%[0-9]+]]:_(p1) = COPY $vgpr0_vgpr1
+    ; CHECK: [[LOAD:%[0-9]+]]:_(s64) = G_LOAD [[COPY]](p1) :: (load (s64), addrspace 1)
+    ; CHECK: [[SEXT_INREG:%[0-9]+]]:_(s64) = G_SEXT_INREG [[LOAD]], 8
+    ; CHECK: $vgpr0_vgpr1 = COPY [[SEXT_INREG]](s64)
+    %0:_(p1) = COPY $vgpr0_vgpr1
+    %1:_(s64) = G_LOAD %0 :: (load (s64), align 8, addrspace 1)
+    %2:_(s64) = G_SEXT_INREG %1, 8
+    $vgpr0_vgpr1 = COPY %2
+...