[JumpThreading] Look through freeze in getPredicateAt() fold

This code is valid for any icmp, so we can safely look through a
freeze when trying to find one.

A caveat here is that replaceFoldableUses() may not end up replacing
any uses in this case. It might make sense to use the freeze as the
context instruction (rather than the terminator) if there is a
freeze, to ensure that it always gets folded. This would require
some changes to how replaceFoldedUses() works though, as it
currently assumes that the value is valid at the end of the block.