Re-apply "[JumpThreading] Ignore free instructions"

It seems the crashes we saw wasn't caused by this (see comments on the review).

> This is basically D108837 but for jump threading. Free instructions
> should be ignored for the threading decision. JumpThreading already
> skips some free instructions (like pointer bitcasts), but does not
> skip various free intrinsics -- in fact, it currently gives them a
> fairly large cost of 2.
>
> Differential Revision: https://reviews.llvm.org/D110290

This reverts commit 4604695d.