MIR Codegen: Phi Elimination Pass

## Summary Remove phi nodes from SSA MIR and convert to an assignment form suitable for stack allocation, using parallel copy insertion and coalescing. **Parent issue:** #687 ## Context SSA form uses φ (phi) nodes to merge values at control flow join points. The EVM has no direct equivalent—we need to convert phis into explicit move/copy instructions inserted at block boundaries. Example: ``` ; Before (SSA with phi) entry: v0 = const 1 branch cond, then_block, else_block then_block: v1 = const 10 jump merge else_block: v2 = const 20 jump merge merge: v3 = phi [v1, then_block], [v2, else_block] ; ← phi node return v3 ; After (phi eliminated) then_block: v1 = const 10 v3 = move v1 ; ← inserted copy jump merge else_block: v2 = const 20 v3 = move v2 ; ← inserted copy jump merge merge: return v3 ``` ## Tasks ### Parallel copy insertion - [ ] For each block with phi nodes, for each predecessor `P`: - Generate assignments `phi_result := incoming_from_P` - Insert as parallel copy sequence at end of `P` (before terminator) ### Parallel copy resolution - [ ] Transform parallel copies into sequential `Move` instructions - [ ] Handle cycles via temporary values (e.g., swap `a, b = b, a` needs a temp) - [ ] Coalesce moves when possible (no conflict) ### Phi removal - [ ] Replace all phi result uses with the new value sources - [ ] Delete phi instructions from MIR - [ ] Update MIR invariants documentation ## Patterns to follow **From Sonatina:** - Simple phi elimination pass with parallel copies at block entries/predecessor exits - Cycle-breaking using temp register/value **From Venom:** - Post-phi form convenient for stack allocation: all values now have standard instruction definitions ## Acceptance Criteria - [ ] MIR pretty-printer shows no `Phi` instructions post-pass - [ ] Tests for diamonds, loops, nested conditionals retain semantic equivalence - [ ] Specific tests for phi cycles (e.g., swap of two values across branches) ## Estimated Complexity **Medium** - Well-understood algorithm, but cycle handling requires care ## Dependencies - #694 (Liveness analysis) - recommended for optimization but not strictly required - MIR structure (done)