An improvement to IR ValueTracking on Non-negative Integers.

Current IR ValueTracking does not recognize the cases below, thus hiding optimization opportunities.

- Induction variable starting from a non-negative integer and incrementing in each iteration is known to be non-negative.
- Result of left shifting a non-negative integer with nsw flag is known to be non-negative.

This change is a small improvement to computeKnownBits on PHI node and Shift.

Patch by Ruben Perez.