Ret from for-loop block sometimes generated bad LLVM bitcode

[marijnh]

This is somewhat subtle, in that I haven't been able to reproduce it in small toy programs, but I had to make several changes to loops to make things compile as part of the switch to new-style loops (#1619). The error you get is about an unterminated block.

See for example

c902eaf#L1R92
c902eaf#L49R1649