Revert "Revert "[IndVars] Replace PHIs if loop exits on 1st iteration""

This reverts commit 6fec6552.

The patch was reverted on incorrect claim that this patch may break LCSSA form
when the loop is not in a simplify form. All IndVars' transform insure that
the loop is in simplify and LCSSA form, so if it wasn't broken before this
transform, it will also not be broken after it.