Revert "[JumpThreading] Ignore free instructions"
It caused compiler crashes, see comment on the code review for repro.
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 1e3c6fc7cb9d2ee6a5328881f95d6643afeadbff.