peel loops with runtime small trip counts

For Hexagon, peeling loops with small runtime trip count is beneficial for our
benchmarks. We set PeelCount in HexagonTargetInfo.cpp and we use PeelCount set
by the target for computing the desired peel count.

Differential Revision: https://reviews.llvm.org/D44880

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@328854 91177308-0d34-0410-b5e6-96231b3b80d8