JIT: improve transition from 32-bit to 64-bit jumps

Use a recursive call to try again, instead of trying to reset local
state. The reset-local-state variant is definitely broken in
some caes, though I could not provoke the JIT buffer overflow
that I was hoping to fix with this change.
update bytecode compiler/optimizer overview in source comments

Explain the recently added "letrec_check" pass.