zombie constants/types are merged with normal constants/types

[khyperia]

fn f(x: bool, y: bool) -> bool {
    x && y
}

causes "checked sub is not supported yet" because it uses a false constant, which is merged with a zombie false constant, which is zombied here

rust-gpu/crates/rustc_codegen_spirv/src/builder/builder_methods.rs

Line 694 in c16ef16

result.1.def(self),