-fsanitize=cfi-icall leads to eliminated icalls with optimization levels above -O0

[jwillbold]

-fsanitize=cfi-icall does not emit CFI statements and eliminates the icall during code elimination, leading to broken binaries.

We have created a minimal example highlighting the issue. The following code gets compiled using clang 19.1.0 x86_64 and -O3 -fsanitize=cfi-icall -flto

void (*pFunc)();

int main(int argc, char** argv) {
    pFunc = argv[1];
    pFunc();
}

to

main:
 mov    rax,QWORD PTR [rsi+0x8]
 mov    QWORD PTR [rip+0x2edd],rax        # 4018 <pFunc>
 ud1    eax,DWORD PTR [eax+0x2]

When setting the -O0, it does emit the icall as expected. We created a compiler explorer example: https://godbolt.org/z/P71hx6xqs

However, we have also found that initializing the pFunc seems to stop the code elimination:

void test() {
    test();
}

//void (*pFunc)(); // Bad
void (*pFunc)() = &test; // Good

int main(int argc, char** argv) {
    pFunc = (void (*)())**argv;
    pFunc();
}

In that case the icall exists. Compiler explorer: https://godbolt.org/z/36hYWb7YW

We have also tested the previous clang version but cannot find a version where this compiles as expected.