Use TopTTI->getGEPCost from within getUserCost

The implementation of getUserCost had duplicated (and hard-coded) the default
logic in getGEPCost. Instead, it is better to use getGEPCost directly, which
limits the default logic to the implementation of one function, and allows
targets to override the behavior.

No functionality change intended.
Add some additional fields to TTI::UnrollingPreferences

In preparation for an upcoming commit implementing unrolling preferences for
x86, this adds additional fields to the UnrollingPreferences structure:

 - PartialThreshold and PartialOptSizeThreshold - Like Threshold and
   OptSizeThreshold, but used when not fully unrolling. These are necessary
   because we need different thresholds for full unrolling from those used when
   partially unrolling (the full unrolling thresholds are generally going to be
   larger).

 - MaxCount - A cap on the unrolling factor when partially unrolling. This can
   be used by a target to prevent the unrolled loop from exceeding some
   resource limit independent of the loop size (such as number of branches).

There should be no functionality change for any in-tree targets.
Implement X86TTI::getUnrollingPreferences

This provides an initial implementation of getUnrollingPreferences for x86.
getUnrollingPreferences is used by the generic (concatenation) unroller, which
is distinct from the unrolling done by the loop vectorizer. Many modern x86
cores have some kind of uop cache and loop-stream detector (LSD) used to
efficiently dispatch small loops, and taking full advantage of this requires
unrolling small loops (small here means 10s of uops).

These caches also have limits on the number of taken branches in the loop, and
so we also cap the loop unrolling factor based on the maximum "depth" of the
loop. This is currently calculated with a partial DFS traversal (partial
because it will stop early if the path length grows too much). This is still an
approximation, and one that is both conservative (because it does not account
for branches eliminated via block placement) and optimistic (because it is only
recording the maximum depth over minimum paths). Nevertheless, because the
loops that fit in these uop caches are so small, it is not clear how much the
details matter.

The original set of patches posted for review produced the following test-suite
performance results (from the TSVC benchmark) at that time:
  ControlLoops-dbl - 13% speedup
  ControlLoops-flt - 15% speedup
  Reductions-dbl - 7.5% speedup