Try to reuse cleanup code when appropriate

[marijnh]

Each ret, break. cont, and maybe others, currently outputs it own block-unwind-cleanup code. A block (or nested set of blocks) which requires non-trivial cleanup code, having a bunch of rets/breaks in it will cause way too much repetitive code to be generated. The compiler will eventually have to notice this situation and somehow centralize the cleanup code.

[brson]

This problem has been greatly compounded by unwinding.