Add BranchProbabilityInfo::releaseMemory to clear the Weights field.

BranchProbabilityInfo was leaking 3MB of memory when running 'opt -O2 verify-uselistorder.lto.bc'.  This was due to the Weights member not being cleared once the pass is no longer needed.

This adds the releaseMemory override to clear that field.  The other fields are cleared at the end of runOnFunction so can stay there.

llvm-svn: 238462