[Docs] Add a performance document.

Summary:
Add a document which describes:

- GEMM performance comparison.
- An experiment that measures the compile time impact
  of enabling Polly when compiling LLVM+Clang+Polly.

Contributed-by: Theodoros <Theodoridis&lt;theodoros.theodoridis@inf.ethz.ch>
Differential Revision: https://reviews.llvm.org/D38330

llvm-svn: 314419