I regularly struggle reproducing failures in greedy due to changes
in priority when resuming the allocation from MIR vs. a complete
compilation starting at IR. That is, the fix in
e0919b189bf2df4f97f22ba40260ab5153988b14 did not really fix the
problem of the instruction distance mattering.
Add a way to bypass all of the priority heuristics for MIR tests,
by prioritizing only by virtual register number. Could also
give this a more specific name, like PrioritizeLowVirtRegNumber
