[MachineScheduler] Add itinerary to schedcover.py. Make default work in the command line filter

Summary:
This patch adds itinerary support to the schedcover.py script. I've been trying to use this script to figure out why SSE and AVX instructions are ending up in separate tablegen scheduler classes and sometimes its because we are using different itineraries.

Rather than using None to indicate the default scheduler model, I now use the string "default". I had to hack around the sorting a little to keep "default" at the beginning. But this also makes it so you can specify "default" on the command line to just get the defaults

I also fixed the regular expression code so that the no_default wasn't evaluated twice.

Reviewers: RKSimon, atrick, jmolloy, javed.absar

Reviewed By: javed.absar

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D44834

llvm-svn: 328608