Loop Unroll: add options and tweak to make Partial unrolling more useful

1. Add FullUnrollMaxCount option that works like MaxCount, but also limits
   the unroll count for fully unrolled loops. So if a loop has an iteration
   count over this, it won't fully unroll.
2. Add CLI options for MaxCount and the new option, so they can be tested
   (plus a test).
3. Make partial unrolling obey MaxCount.

An example use-case (the out of tree one this is originally designed for) is
a target’s TTI can analyze a loop and decide on a max unroll count separate
from the size threshold, e.g. based on register pressure, then constrain
LoopUnroll to not exceed that, regardless of the size of the unrolled loop.

llvm-svn: 265562