[utils] Add an '--only-merge' option to the code coverage prep script

In --only-merge mode, the script terminates after the profile merging
step.  This makes the script less stateful: it's more natural to split
the merge out into a separate step instead of relying on the first
invocation of the script to do it.

This should not break any existing users of the script.

llvm-svn: 285247