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.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@202853 91177308-0d34-0410-b5e6-96231b3b80d8
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.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@202854 91177308-0d34-0410-b5e6-96231b3b80d8