fix(codegen): prevent index out of bounds in dead_storage reaching_definitions

[ArshLabs]

What panics and where

solang compile --target polkadot panics with:

thread 'main' panicked at src/codegen/dead_storage.rs:376:16:
index out of bounds: the len is 0 but the index is 0

The panic is in apply_transfers() at the line:

*vars = res[0].clone();

res is built by iterating over transfers in the first pass (lines 343–371). When a basic block has zero instructions — which happens with a Yul for loop whose body contains only continue — transfers is empty, res is never pushed to, and indexing res[0] panics.

Root cause: two cooperating bugs

Bug 1 — dead_storage.rs: apply_transfers() does not guard against an empty transfers/res before indexing res[0]. The fix is a four-line early-return that records the current reaching-definition set for the block and returns without touching *vars, which is the correct semantic: an empty block has no transfers, so the outgoing state equals the incoming state.

Bug 2 — sema/yul/for_loop.rs: resolve_for_loop() computed post-block reachability purely from whether the body block fell through (resolved_exec_block.1). A body that ends with a reachable continue always reaches the post block but never falls through, so post-block reachability was incorrectly set to false. Codegen then skipped the post block's instructions entirely, producing an empty basic block with zero instructions — the direct trigger for Bug 1. The fix adds a helper (block_ends_with_reachable_continue) that walks the body's last reachable statement and ORs its result into the post-block reachability flag.

Changes

• src/codegen/dead_storage.rs — guard apply_transfers() against empty transfers/res.
• src/sema/yul/for_loop.rs — fix post-block reachability when body ends with a reachable continue; add block_ends_with_reachable_continue and statement_ends_with_reachable_continue helpers.
• tests/polkadot_tests/yul.rs — regression test continue_only_for_body that reproduces the MRE from the issue.

Minimum reproducer

contract C {
    function f() public pure {
        assembly {
            for { let i := 0 } lt(i, 1) { i := add(i, 1) } { continue }
        }
    }
}

solang compile --target polkadot mre.sol

Before this fix: compiler panics. After this fix: compiles cleanly.

Fixes #1876