ScopDetection: Only detect scops that have at least one read and one write

ScopDetection: Only detect scops that have at least one read and one write

Scops that only read seem generally uninteresting and scops that only write are
most likely initializations where there is also little to optimize.  To not
waste compile time we bail early.

Differential Revision: http://reviews.llvm.org/D7735

llvm-svn: 229820