GlobalISel: Guard return in llvm::getIConstantSplatVal (#71989)

getIConstantVRegValWithLookThrough could return NULL.

llvm/lib/CodeGen/GlobalISel/Utils.cpp

@@ -1116,9 +1116,9 @@ std::optional<APInt>
 llvm::getIConstantSplatVal(const Register Reg, const MachineRegisterInfo &MRI) {
   if (auto SplatValAndReg =
           getAnyConstantSplat(Reg, MRI, /* AllowUndef */ false)) {
-    std::optional<ValueAndVReg> ValAndVReg =
-        getIConstantVRegValWithLookThrough(SplatValAndReg->VReg, MRI);
-    return ValAndVReg->Value;
+    if (std::optional<ValueAndVReg> ValAndVReg =
+        getIConstantVRegValWithLookThrough(SplatValAndReg->VReg, MRI))
+      return ValAndVReg->Value;
   }
 
   return std::nullopt;

llvm/test/CodeGen/AMDGPU/GlobalISel/amdgpu-prelegalizer-combiner-crash.mir

@@ -0,0 +1,26 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 3
+# RUN: llc -march=amdgcn -mcpu=gfx1030 -run-pass amdgpu-prelegalizer-combiner %s -o - | FileCheck -check-prefix=GCN %s
+
+---
+name:            non_inlineable_imm_splat
+body:             |
+  bb.1:
+    liveins: $vgpr0
+
+    ; GCN-LABEL: name: non_inlineable_imm_splat
+    ; GCN: liveins: $vgpr0
+    ; GCN-NEXT: {{  $}}
+    ; GCN-NEXT: [[COPY:%[0-9]+]]:_(<2 x s16>) = COPY $vgpr0
+    ; GCN-NEXT: [[C:%[0-9]+]]:_(s16) = G_FCONSTANT half 0xH4200
+    ; GCN-NEXT: [[BUILD_VECTOR:%[0-9]+]]:_(<2 x s16>) = G_BUILD_VECTOR [[C]](s16), [[C]](s16)
+    ; GCN-NEXT: [[SUB:%[0-9]+]]:_(<2 x s16>) = G_SUB [[COPY]], [[BUILD_VECTOR]]
+    ; GCN-NEXT: $vgpr0 = COPY [[SUB]](<2 x s16>)
+    ; GCN-NEXT: SI_RETURN implicit $vgpr0
+    %0:_(<2 x s16>) = COPY $vgpr0
+    %2:_(s16) = G_FCONSTANT half 0xH4200
+    %1:_(<2 x s16>) = G_BUILD_VECTOR %2(s16), %2(s16)
+    %3:_(<2 x s16>) = G_SUB %0, %1
+    $vgpr0 = COPY %3(<2 x s16>)
+    SI_RETURN implicit $vgpr0
+
+...