Fix bug #226: the library predicate lazy.read_if_val/2 was incorrectly declared
as pure.  The reason for it being impure rather than semipure is to avoid any
goals containing calls to force/1 being reordered with respect to it.

library/lazy.m:
    Make read_if_val/2 as impure.

deep_profiler/autopar_types.m:
     Confrom to the above change.
Merge branch 'master' of https://github.com/Mercury-Language/mercury