[DebugInfo][InstrRef][NFC] Bypass a frequently-noop loop

Bypass this loop if it would do nothing -- if there are no register masks
to be examined, there's no point looking at each location to see if the
location has been def'd. Awkwardly, this was responsible for almost an
entire half a percent of performance improvement on CTMark.

Differential Revision: https://reviews.llvm.org/D118613