[m-dev.] diff for review

[Zoltan Somogyi]

> compiler/code_info.m
> 	Fix a code generator abort for if-then-elses with conditions
> 	that cannot succeed by having code_info__generate_branch_end
> 	do nothing if the instmap is unreachable.

I believe there should be no need to modify code_info; the proper
fix seems to me to be to put the relevant variables in the then-part's
post-birth set. Read the comment at the top of liveness.m.

Since liveness is supposed to do this already, the bug seems to be
in liveness.m.

Zoltan.