[flang] Fix buildbot (new warnings on old code)

The clang-aarch64-full-2stage buildbot is complaining about a
warning with three instances in f18 code (none modified recently).
The warning is for using the | bitwise OR operator on bool operands.

In one instance, the bitwise operator was being used instead of the
logical || operator in order to avoid short-circuting.  The fix
requires using some temporary variables.  In the other two instances,
the bitwise operator seemed more idiomatic in context, but can be
replaced without harm with the logical operator.

Pushing without review as confidence is high and nobody wants
a buildbot to stay sad for long.