IndVarSimplify: Allow LFTR to fire more often

I added a pessimization in r217102 to prevent miscompiles when the
incremented induction variable was used in a comparison; it would be
poison.

Try to use the incremented induction variable more often when we can be
sure that the increment won't end in poison.

Differential Revision: http://reviews.llvm.org/D6222

llvm-svn: 222213