Kill IRBuilder::blockIsIncompatible (try 2)

Summary: The only purpose of IRBuilder::blockIsIncompatible left was to prevent
loops when Eval.JitLoops is off.  This diff gets rid of that, and
translateRegion now properly follows the region description passed to
it.  If any region former doesn't want to expose loops, it should not
include backedge arcs in the produced region.

Reviewed By: @bertmaher

Differential Revision: D1457757