[InstSimplify][CaptureTracking] Reduce scope of special case

As described in PR54002, the "icmp of load from global" special
case in CaptureTracking is incorrect, as is the InstSimplify code
that makes use of it.

This moves the special case from CaptureTracking into InstSimplify
to limit its scope to the buggy transform only, and not affect
other code using CaptureTracking as well.