[JumpThreading] Simplify getPredicateAt() based folding

It's sufficient to just fold the icmp to true/false here, and then
let constant terminator folding take care of the rest.

It should be noted that while replaceFoldableUses() may not replace
all uses of the icmp, at least the use in the terminator we're
working on is always replaceable, so terminator constant folding
should be reliably enabled as a subsequent step.