Introduce PollyIRBuilder

PollyIRBuilder is currently just a typedef to IRBuilder<>. Consequently, this
change should not affect behavior. In subsequent patches we will extend its
functionality to emit loop.parallel metadata.
Emit llvm.loop metadata for parallel loops

For now we only mark innermost loops for the loop vectorizer.  We could later
also mark not-innermost loops to enable the introduction of openmp parallelism.