Silence 'variable unused' warning in release mode

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@177515 91177308-0d34-0410-b5e6-96231b3b80d8
Add option -polly-code-generator=none

This allows us to test Polly and the Polly optimizer without actually doing
code generation at the end. By enabling this option, we can also measure the
compile time overhead due to code generation and the cost of LLVM optimizing the
newly generated code.t

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@177516 91177308-0d34-0410-b5e6-96231b3b80d8