Introduce runtime unrolling disable matadata and use it to mark the scalar loop from...

Introduce runtime unrolling disable matadata and use it to mark the scalar loop from vectorization.

Runtime unrolling is an expensive optimization which can bring benefit
only if the loop is hot and iteration number is relatively large enough.
For some loops, we know they are not worth to be runtime unrolled.
The scalar loop from vectorization is one of the cases.

llvm-svn: 231631