Use estimated number of optimized insns in unroll-threshold computation.

Use estimated number of optimized insns in unroll-threshold computation.

If complete-unroll could help us to optimize away N% of instructions, we
might want to do this even if the final size would exceed loop-unroll
threshold. However, we don't want to unroll huge loop, and we are add
AbsoluteThreshold to avoid that - this threshold will never be crossed,
even if we expect to optimize 99% instructions after that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228434 91177308-0d34-0410-b5e6-96231b3b80d8