[InstCombine] Guard against many users when swapping icmp operands

This addresses the compile-time regression reported on D144369.
If we don't fold constant operands early, then we might end up
walking very large use lists of constants here. Explicitly exclude
constants, and also limit the number of inspected users to avoid
degenerate cases like this.

This entire transform shouldn't be part of InstCombine in the
first place though.