SSA2: Convenience functions for finding 'main'
Convert the SSA2 type-checker to the multi-entry SSA2 IL

Changes were almost exactly like:
251bd86 Convert SSA type checker to multi-entry SSA (partially)

This is change still leaves out rules that are specific to multi-entry
functions (such as making sure that every entry is a root in the
dominator forest).

Type checking profiling statements is still incomplete.