Bugfix: ScalarEvolution incorrectly assumes that the start of certain

add recurrences don't overflow.

This change makes the optimization more restrictive.  It still assumes
that an overflowing `add nsw` is undefined behavior; and this change
will need revisiting once we have a consistent semantics for poison
values.

Differential Revision: http://reviews.llvm.org/D7331

llvm-svn: 228552