RegScavenger::determineKillsAndDefs was calling BitVector::clear() which
sets the size of the BitVector to 0. Changed to reset() which un-sets the
contents of the vector but doesn't change the size.
This code started tripping an assert after a change I'm working on in BitVector.
I haven't included a test because I have no idea how to exercise this code.
Not sure who to send this review to, looks like this bug was introduced in 2014 and doesn't get much attention.
I assume if you put an unreachable here, something breaks? If you find that testcase, it should be easy to derive one