gce: don't ICE on non-local const
Fixesrust-lang/rust#133808
I have absolutely no idea what I'm doing here, but I followed `@BoxyUwU` 's [instructions](https://github.com/rust-lang/rust/issues/133808#issuecomment-3009122957), and turns out this small change fixesrust-lang/rust#133808, and doesn't seem to break anything else.
(This code path is only reachable when the GCE feature gate is enabled, so even if it does break in a way that is not caught by current test coverage, I guess it's not as bad as breaking stable or non-incomplete features?)
Anyways, r? `@BoxyUwU` , if you don't mind.