[LV] Allow a scalable VF for the epilogue.

For some reason we limited the epilogue VF to be fixed-width, but there
is not necessarily a reason for doing so. If the main VF=vscale x 16, the
epilogue VF could be either fixed-width, or a scalable VF upto vscale x 8.

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D118688