Consider structurally impossible Sized predicates in MIR

[qaijuang]

Fixes #156051

ImpossiblePredicates filters predicates mentioning local generics before invoking the trait solver, which avoids normalization and solver work in generic contexts, thereby skipping predicates such as dyn Trait<T>: Sized, even though they are impossible for every substitution.

This PR adds a narrow structural fast path for positive Sized predicates whose self type is known-unsized without normalization, letting the pass clear unreachable bodies before later MIR opts can evaluate SizedTypeProperties constants such as size_of::<Self>().

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[rustbot]

r? @TaKO8Ki

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[qaijuang]

cc @matthiaskrgr