Allow to run the Polly preopt passes with -O0

To extract a preoptimized LLVM-IR file from a C-file run:

clang -Xclang -load -Xclang LLVMPolly.so -O0 -mllvm -polly file.c -S -emit-llvm

On the generated file you can directly run passes such as:
'opt -view-scops file.s'

llvm-svn: 146560