Reduce false positives in printf/scanf format checker

Summary:
The printf/scanf format checker is a little over-zealous in handling the conditional operator.  This patch reduces work by not checking code-paths that are never used and reduces false positives regarding uncovered arguments, for example in the code fragment:

printf(minimal ? "%i\n" : "%i: %s\n", code, msg);

Reviewers: rtrieu

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D15636

llvm-svn: 262025