[SCEV] Improve AddRecs' range computation in Expensive Range Sharpening mode

Apply loop guards to AddRec's start in range computation for
non-self-wrapping AddRecs.

According to CT measurements, this has a wide negative compile time impact,
so we hold it in expensive range sharpening mode where it's not so critical.
However, we need to find a way to share benefits of this mode with default mode.

Patch by Aleksandr Popov!

Differential Revision: https://reviews.llvm.org/D147557
Reviewed By: mkazantsev