[SimplifyLibCalls] Use inbounds GEP

[SimplifyLibCalls] Use inbounds GEP

When converting strchr(p, '\0') to p + strlen(p) we know that
strlen() must return an offset that is inbounds of the allocated
object (otherwise it would be UB), so we can use an inbounds GEP.
An equivalent argument can be made for the other cases.