swift234

Revert "Revert "SimplifyCFG: Don't count 'free' instructions during jump-threading""

Reinstantiate rr22712. Now that updating SSA form is more robust we should not
run into troubles anymore.

Original commit message:

"Use the inliner's heuristic to decide which instructions are 'free' (constants,
etc).

We were not jumpthreading a block because it had four instruction in it - two of
them integer_literals."

rdar://18594600

Swift SVN r22761