Fix branch coverage merging in FunctionCoverageSummary::get() for instantiation

Fix branch coverage merging in FunctionCoverageSummary::get() for instantiation
groups.

This change corrects the implementation for the branch coverage
summary to do the same thing for branches that is done for lines and regions.
That is, across function instantiations in an instantiation group, the maximum
branch coverage found in any of those instantiations is returned, with the
total number of branches being the same across instantiations.

Differential Revision: https://reviews.llvm.org/D102193