## Problem
Exit PHI generation was using header_id as predecessor, but when
build_expression(condition) creates new blocks, the actual branch
instruction is emitted from a different block, causing:
"phi pred mismatch: no input for predecessor BasicBlockId(X)"
## Solution
- Modified build_exit_phis() to accept branch_source_block parameter
- Capture actual block after condition evaluation in loop_builder.rs
- Use branch_source_block instead of header_id for PHI inputs
## Progress
- Error changed from ValueId(5941)/BasicBlockId(4674) to
ValueId(5927)/BasicBlockId(4672), showing partial fix
- Added comprehensive test suite in mir_loopform_exit_phi.rs
- Added debug logging to trace condition block creation
## Status
Partial fix - unit tests pass, but Test 2 (Stage-B compilation) still
has errors. Needs further investigation of complex nested compilation
scenarios.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
