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

[lapla-cogito]

fix #141400, fix #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, we've added a guard that only applies when non-regional inference variables remain.

[rustbot]

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