Fix another case where loop-convert wasn't handling correctly data members.

Summary:
If the container expression was obtained from the point where "size" (which usually is a const method) is invoked, then the topmost node in this expression may be an implicit cast to const.

When the container is a data member, the check was trying to obtain the member expression directly and was failing in the case mentioned above. This is solved by ignoring implicit casts.

Reviewers: klimek

Subscribers: cfe-commits, alexfh

Differential Revision: http://reviews.llvm.org/D14378

llvm-svn: 252278