genhtml: Reduce load times for complex coverage data files

genhtml uses a significant amount of time loading coverage data files
containing complex branch coverage data (i.e. data with a large number
of branches per basic block). Most of this time is spent storing
branch coverage data in a vector-based data representation, with an
unnecessary amount of cross-checking being done for existing branch
data.

Fix this by replacing the vector based data representation by two
separate representations, scalar for storage and hash for processing,
and by moving cross-checking out of the hot path. This results in a
significant speedup at the cost of a minor increase in memory usage.

Test results for "make -C genhtml_output/ SIZE=large":

Original:
  6 tests executed, 6 passed, 0 failed, 0 skipped (time 768.4s, mem
  893.8MB)

Patched:
  6 tests executed, 6 passed, 0 failed, 0 skipped (time 202.3s, mem
  908.10MB)

Signed-off-by: Peter Oberparleiter <[email protected]>

Author: oberpar