InstrProf: Don't keep a large sparse list around just to zero it

The Terms vector here represented a polynomial of of all possible
counters, and is used to simplify expressions when generating coverage
mapping. There are a few problems with this:

1. Keeping the vector as a member is wasteful, since we clear it every
   time we use it.
2. Most expressions refer to a subset of the counters, so we end up
   iterating over a large number of zeros doing nothing a lot of the
   time.

This updates the user of the vector to store the terms locally, and
uses a sort and combine approach so that we only operate on counters
that are actually used in a given expression. For small cases this
makes very little difference, but in cases with a very large number of
counted regions this is a significant performance fix.