Make ISLE dataflow optimization control-flow aware (BrIf/BrTable)

[avrabe]

Context

PR #53 eliminated the has_unsupported_isle_instructions skip block — all standard WASM instructions are now wired into the ISLE pipeline. However, functions containing BrIf or BrTable are still skipped by has_dataflow_unsafe_control_flow for the two term-rewriting passes (optimize_term_rewriting and optimize_advanced_instructions).

Problem

The ISLE dataflow optimization (simplify_with_env) can incorrectly move side effects outside of control flow blocks. For example, a LocalSet after a BrIf may be moved to execute unconditionally — a semantic violation.

This means functions with conditional branches don't get term-rewriting optimizations (constant propagation through locals, redundant load elimination, etc.), even though they now get all other passes (DCE, branch simplification, code folding, etc.).

Solution

Make the dataflow analysis control-flow aware:

1. Track which values are defined inside conditional blocks
2. Don't propagate LocalSet values across branch boundaries
3. Consider using dominator tree analysis to determine safe propagation points
4. Or: split the function into basic blocks and optimize each independently

Files

• loom-core/src/lib.rs: has_dataflow_unsafe_control_flow (line ~6654), optimize_term_rewriting (line ~6689), optimize_advanced_instructions (line ~10700)

Impact

This is the last major optimization coverage gap. Removing this restriction would give every function full optimization, not just functions without conditional branches.