[CodeGen] Skip null physical register in AntiDepBreaker (NFCI)

D151036 adds an assertions that prohibits iterating over sub- and
super-registers of a null register. This is already the case when
iterating over register units of a null register, and worked by
accident for sub- and super-registers.
The only place where the assertion is currently triggering is in
CriticalAntiDepBreaker::ScanInstruction. Other places are changed
in case new assertions are added and should be harmless otherwise.