[GMR] Switch to a DenseMap and clean up the iteration loop. NFC.

Since we have to iterate this map not that infrequently, we should use
a map that is efficient for iteration. It is also almost certainly much
faster for lookups as well. There is more to do in terms of reducing the
wasted overhead of GMR's runtime though. Not sure how much is worthwhile
though.

The loop improvements should hopefully address the code review that
Duncan gave when he saw this code as I moved it around.

llvm-svn: 242891