[new docs] Performance Tips for Frontend Authors

As mentioned on llvm-dev, this is a new documentation page intended to collect tips for frontend authors on how to generate IR that LLVM is able to optimize well. These types of things come up repeated in review threads and it would be good to have a place to save them.

I added a small handful to start us off, but I mostly want to get the framework in place. Once the docs are here, we can add to them incrementally.  If you know of something appropriate for this page, please add it!

Differential Revision: http://reviews.llvm.org/D7890

llvm-svn: 230807