RegAllocGreedy: Un-disable test in expensive_checks builds
This reverts a8f3ebaf11c3745e5123054776eb71755d16f2f9. You need to
use -verify-regalloc to get a MachineVerifier run with LiveIntervals,
otherwise cases not covered by the basic liveness implementation
in the verifier are passed through (which covers most use of undefined
subrange errors).
