[Bug tree-optimization/89478] missed optimization for lambda expression when variable is uninitialized when declared

[SztfG at yandex dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89478

--- Comment #3 from SztfG at yandex dot ru ---
Another testcase:
int test4 = []() constexpr {int a = a; a = 5; return a;}();

GCC is able compile this, so it "think" this is valid constexpr lambda, but
anyway doing this:

_GLOBAL__sub_I_test4:
        movl    $5, test4(%rip)
        ret
test4:
        .zero   4