perf: cache successor params to avoid O(P×A) cloning in arg demotion

[Fibonacci747]

demote_block_signature called get_succ_params() inside a nested loop over predecessors and demotable arguments. Each call cloned the entire arguments vector just to access a single element by index. With P predecessors, A demotable args, and N total args, this resulted in P×A×N unnecessary Value copies instead of P×N.

Moved get_succ_params() call outside the inner loop and cache the result, reducing vector cloning from O(P×A) to O(P) per block.

[fuel-cla-bot]

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PR Summary

Low Risk
Small, localized performance change that only alters how often get_succ_params() is called; functional behavior should remain unchanged aside from potential indexing/panic if assumptions were previously masked.

Overview
Improves the arg-demotion optimization by caching a predecessor block’s successor parameters (get_succ_params) once per predecessor in demote_block_signature, instead of re-fetching/cloning them for each demoted argument.

This reduces redundant Vec<Value> cloning from O(P×A) to O(P) per block while keeping IR transformation behavior the same.

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Cached params causes silent no-op replacement for duplicate values

Medium Severity

When the same Value is passed at multiple demotable argument positions from the same predecessor (e.g., br block(V, V)), caching get_succ_params before the inner loop causes incorrect behavior. The first iteration's replace_values replaces ALL occurrences of V in the terminator with get_local_val_0. On the next iteration, replace_values with {V → get_local_val_1} becomes a silent no-op since V no longer exists in the terminator. The result is the terminator passing the wrong GetLocal pointer for subsequent duplicate positions.