Implement TTI getUnrollingPreferences for PowerPC

The PowerPC A2 core greatly benefits from aggressive concatenation unrolling;
use the new getUnrollingPreferences to enable this by default when targeting
the PPC A2 core.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190549 91177308-0d34-0410-b5e6-96231b3b80d8