[GlobalOpt] Handle non-instruction MTI source (PR54572)

This was reusing a cast to GlobalVariable to check for an
Instruction, which means we'll try to dereference a null pointer
if it's not actually a GlobalVariable. We should be casting
MTI->getSource() instead.

I don't think this problem is really specific to opaque pointers,
but it certainly makes it a lot easier to reproduce.

Fixes #54572.

llvm/lib/Transforms/IPO/GlobalOpt.cpp

@@ -230,7 +230,7 @@ CleanupPointerRootUsers(GlobalVariable *GV,
       if (MemSrc && MemSrc->isConstant()) {
         Changed = true;
         MTI->eraseFromParent();
-      } else if (Instruction *I = dyn_cast<Instruction>(MemSrc)) {
+      } else if (Instruction *I = dyn_cast<Instruction>(MTI->getSource())) {
         if (I->hasOneUse())
           Dead.push_back(std::make_pair(I, MTI));
       }

llvm/test/Transforms/GlobalOpt/pr54572.ll

@@ -0,0 +1,23 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --check-globals
+; RUN: opt -S -globalopt < %s | FileCheck %s
+
+@b = internal global ptr null
+@c = internal global [2 x ptr] zeroinitializer
+
+declare void @llvm.memcpy.p0.p0.i64(ptr noalias nocapture writeonly, ptr noalias nocapture readonly, i64, i1 immarg)
+
+;.
+; CHECK: @[[B:[a-zA-Z0-9_$"\\.-]+]] = internal unnamed_addr global ptr null
+; CHECK: @[[C:[a-zA-Z0-9_$"\\.-]+]] = internal unnamed_addr constant [2 x ptr] zeroinitializer
+;.
+define void @test() {
+; CHECK-LABEL: @test(
+; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr @b, ptr getelementptr inbounds ([2 x ptr], ptr @c, i64 0, i64 1), i64 8, i1 false)
+; CHECK-NEXT:    ret void
+;
+  call void @llvm.memcpy.p0.p0.i64(ptr @b, ptr getelementptr inbounds ([2 x ptr], ptr @c, i64 0, i64 1), i64 8, i1 false)
+  ret void
+}
+;.
+; CHECK: attributes #[[ATTR0:[0-9]+]] = { argmemonly nofree nounwind willreturn }
+;.