[PPC/LoopUnrollRuntime] Don't avoid high-cost trip count computation on the PPC/A2

[PPC/LoopUnrollRuntime] Don't avoid high-cost trip count computation on the PPC/A2

On X86 (and similar OOO cores) unrolling is very limited, and even if the
runtime unrolling is otherwise profitable, the expense of a division to compute
the trip count could greatly outweigh the benefits. On the A2, we unroll a lot,
and the benefits of unrolling are more significant (seeing a 5x or 6x speedup
is not uncommon), so we're more able to tolerate the expense, on average, of a
division to compute the trip count.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237947 91177308-0d34-0410-b5e6-96231b3b80d8