[LoopRotation] Allow loop header duplication if vectorization is forced.

-Oz normally does not allow loop header duplication so this loop wouldn't be
vectorized.  However the vectorization pragma should override this and allow
for loop rotation.

rdar://problem/49281061

Original patch by Adam Nemet.

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D59832