[LoopPredication] Handle the case when the guard and the latch IV have different offsets

This is a follow up change for D37569.

Currently the transformation is limited to the case when:
 * The loop has a single latch with the condition of the form: ++i <pred> latchLimit, where <pred> is u<, u<=, s<, or s<=.
 * The step of the IV used in the latch condition is 1.
 * The IV of the latch condition is the same as the post increment IV of the guard condition.
 * The guard condition is of the form i u< guardLimit.

This patch enables the transform in the case when the latch is

 latchStart + i <pred> latchLimit, where <pred> is u<, u<=, s<, or s<=.

And the guard is

 guardStart + i u< guardLimit

Reviewed By: anna

Differential Revision: https://reviews.llvm.org/D39097

llvm-svn: 316768