Const-fold non-empty span pops

[odupontt]

This completes the existing const-folding for snapshot pops by propagating the success values of span.pop_front() and span.pop_back() when the span is known to be non-empty. The empty case was already optimized and the original implementation left an explicit TODO for the success path, so this change closes a real gap rather than introducing new behavior. Regression tests were added for both non-empty cases.

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Note

Medium Risk
Changes const-folding control-flow rewriting for array_snapshot_pop_front/back success paths, which can affect generated lowering and requires correct materialization of tracked values and snapshot/boxing semantics. Scope is localized to the lowering optimizer and covered by new regression tests.

Overview
Extends const-folding for span.pop_front()/span.pop_back() when the span is known non-empty, rewriting the match to jump directly to the Some arm while also propagating both the remaining span and the popped element.

Adds try_materialize_var_info to synthesize the necessary lowered statements (consts, snapshots, struct/enum construction, boxing, and array rebuild via array_new/array_append) when the optimizer needs real variables for tracked VarInfo values. Updates lowering test data with new regression cases for the non-empty pop_front and pop_back optimizations.

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