[LV] Stop saying "use -Rpass-analysis=loop-vectorize"

This is PR28376.

Unfortunately given the current structure of optimization diagnostics we
lack the capability to tell whether the user has
passed -Rpass-analysis=loop-vectorize since this is local to the
front-end (BackendConsumer::OptimizationRemarkHandler).

So rather than printing this even if the user has already
passed -Rpass-analysis, this patch just punts and stops recommending
this option.  I don't think that getting this right is worth the
complexity.

Differential Revision: https://reviews.llvm.org/D26563

llvm-svn: 286662