RegisterPass: Disable Polly by default

We disable Polly by default and add a new option '-polly' that enables Polly.
This allows us to create an the alias

$ alias clang clang -Xclang -load -Xclang LLVMPolly.so

which loads Polly always into clang. It can now be enabled by running:

$ clang -O3 -mllvm -polly file.c

To enable it by default an alias pollycc can be create

$ alias pollycc clang -O3 -mllvm -polly

llvm-svn: 144917