Lower inlining threshold when the caller has minsize attribute.

When the caller has optsize attribute, we reduce the inlinining threshold
to OptSizeThreshold (=75) if it is not already lower than that. We don't do
the same for minsize and I suspect it was not intentional. This also addresses
a FIXME regarding checking optsize attribute explicitly instead of using the
right wrapper.

Differential Revision: http://reviews.llvm.org/D16493

llvm-svn: 259120