[flang] Make EndProgramStmt a NOP + early return

Fix done in D143055 can be simpler by making EndProgramStmt a NOP
and dealing with the exit in `endNewFunction` in a centralize way.
Also add finalization when there is an early exit in the main
program.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D143065