transmutability: only short-circuit is_transmutable for non-region infers

[lapla-cogito]

fixes #141400, fixes #148809

An ICE occurs when using std::mem::TransmuteForm for trait resolution when it contains inferred variables. This can be resolved by adding an early return condition to is_transmutable. However, since most existing type resolution appears to be handled by rustc_transmute::TransmuteTypeEnv, I've added a guard that only applies when non-regional inference variables remain.

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[lcnr]

It seems wrong to return an error here. It should not be impossible to transmute. We should instead stall somehow here. There should probably just be an Answer::Maybe or sth to deal with cases where there are still inference variable to get the trait solver to try again later