[opt-remarks] If hotness threshold is set, ignore remarks without hotness

These are blocks that haven't not been executed during training.  For large
projects this could make a significant difference.  For the project, I was
looking at, I got an order of magnitude decrease in the size of the total YAML
files with this and r319235.

Differential Revision: https://reviews.llvm.org/D40678

llvm-svn: 319556