Merge blocks before constructing SSA form

This keeps SSA construction from causing a stack overflow for exceptionally large control-flow graphs.

kernel/modelverse_jit/cfg_optimization.py

@@ -522,6 +522,7 @@ def optimize(entry_point, jit):
     optimize_graph_flow(entry_point)
     elide_local_checks(entry_point)
     optimize_graph_flow(entry_point)
+    merge_blocks(entry_point)
     eliminate_trivial_phis(entry_point)
     entry_point = cfg_ssa_construction.construct_ssa_form(entry_point)
     if jit.direct_calls_allowed: