Handle metadata users of old function after legalization

Just replace these users with poison instead of dropping them

Author: scottp101

IGC/AdaptorCommon/LegalizeFunctionSignatures.cpp

@@ -478,8 +478,11 @@ void LegalizeFunctionSignatures::FixFunctionBody(Module &M) {
 
     // Now that all instructions are transferred to the new func, delete the old func
     pFunc->removeDeadConstantUsers();
-    pFunc->dropAllReferences();
-    pFunc->removeFromParent();
+    // There may be cases where the old function was referenced by just metadata.
+    // If we've cleaned up all "real" uses, then we can just update the metadata
+    // with poison so we can delete the function.
+    pFunc->replaceAllUsesWith(PoisonValue::get(pFunc->getType()));
+    pFunc->eraseFromParent();
   }
 }
 

IGC/Compiler/tests/LegalizeFunctionSignatures/metadata-use.ll

@@ -0,0 +1,28 @@
+;=========================== begin_copyright_notice ============================
+;
+; Copyright (C) 2025 Intel Corporation
+;
+; SPDX-License-Identifier: MIT
+;
+;============================ end_copyright_notice =============================
+;
+; RUN: igc_opt --opaque-pointers --igc-legalize-function-signatures -S < %s 2>&1 | FileCheck %s
+; ------------------------------------------------
+; LegalizeFunctionSignatures
+; ------------------------------------------------
+
+; Test that functions with illegal types referenced by metadata are handled correctly.
+
+define spir_kernel void @test_kernel() {
+  call void @foo(i1 true)
+  ret void
+}
+
+define internal void @foo(i1) {
+    ret void
+}
+
+!md = !{!0}
+!0 = !{ptr @foo}
+
+; CHECK: !0 = !{ptr poison}