Bugfix: SCEV incorrectly marks certain add recurrences as nsw

When creating a scev for sext({X,+,Y}), scev checks if the expression
is equivalent to {sext X,+,zext Y}.  If it can prove that, it also
tags the original {X,+,Y} as <nsw>, which is not correct.

In the test case I run `-scalar-evolution` twice because the bug
manifests only once SCEV has run through and seen the `sext`
expressions (and then does a in-place mutation on {X,+,Y}).

Differential Revision: http://reviews.llvm.org/D7495

llvm-svn: 228586