[llvm-cov] Suppress sub-line highlights in simple cases

llvm-cov tends to highlight too many regions because its policy is to
highlight all region entry segments. This can look confusing to users:
not all region entry segments are interesting and deserve highlighting.
Emitting these highlights only when the region count differs from the
line count is a more user-friendly policy.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316109 91177308-0d34-0410-b5e6-96231b3b80d8