`MutationTree.num_mutations` might have been returning the largest mutant IDs

[JonathanFoo0523]

The num_mutations is intiliased and modified as below:

self.num_mutation = 0

# being modified by recursive call to `populate()`
temp: int = functools.reduce(max, self.nodes[node_id].mutation_ids, 0)
self.num_mutations = max(self.num_mutations, temp)

However, self.nodes[node_id].mutation_ids have range [0, largest_mutants_id], thus self.num_mutations <= largest_mutants_id

But in most case, we have self.num_mutations == largest_mutants_id + 1

I think (not verified) that this bug has been present since before the script was modified to accommodate the new format (flatten representation) of the mutation_info_file.

[JonathanFoo0523]

If this is the case, then

unkilled_mutants: Set[int] = set(range(0, mutation_tree.num_mutations))

in llvm_test_suite_runner is wrong and might crash when we do

unkilled_mutants.remove(largest_mutant)