[ValueTracking] Don't set nsw flag for inbounds addition

When computing the known bits for a GEP, don't set the nsw flag
when adding an offset to an address. The nsw flag only applies to
pure offset additions (see also D90708).

The nsw flag is only used in a very minor way by the code, to the
point that I was not able to come up with a test case where it
makes a difference.

Differential Revision: https://reviews.llvm.org/D90637