Enable runtime unrolling with unroll pragma metadata

Enable runtime unrolling with unroll pragma metadata

Enable runtime unrolling for loops with unroll count metadata ("#pragma unroll N")
and a runtime trip count. Also, do not unroll loops with unroll full metadata if the
loop has a runtime loop count. Previously, such loops would be unrolled with a
very large threshold (pragma-unroll-threshold) if runtime unrolled happened to be
enabled resulting in a very large (and likely unwise) unroll factor.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242047 91177308-0d34-0410-b5e6-96231b3b80d8