[IRCE] Add NSW flag to main loop's indvar base

We have guarantees that induction variable will not overflow in the main
loop after the loop constrained. Therefore we can add no wrap flags on
its base in order not to miss info that loop is countable.

Add NSW flag now, since adding NUW flag requires a bit more complicated
analysis.

Reviewed By: skatkov

Differential Revision: https://reviews.llvm.org/D154954