ScalarEvolution: do not set nuw when creating exprs of form <expr> + <all-ones>.

The nuw constraint will not be satisfied unless <expr> == 0.

This bug has been around since r102234 (in 2010!), but was uncovered by
r251052, which introduced more aggressive optimization of nuw scev expressions.

Differential Revision: http://reviews.llvm.org/D14850

llvm-svn: 253627