[PATCH] Add loop unroll pragma support

[Eli Bendersky]

>>! In D4089#8, @meheff wrote:
> Thanks for the suggestions.  They have been addressed.  Pekka (cc'd) on the optimizer patch thread suggested changing the metadata string from  llvm.loopunroll.* to llvm.loop.unroll.*.  I like the suggestion as it partitions the namespace more logically.  It makes sense then similarly to change the existing  llvm.vectorizer.* and llvm.interleave.* names to llvm.loop.vectorizer.* and llvm.loop.interleave.*.  Anybody have an opinion?
> 
> If I do make this change, then there will be a mismatch between the emitted metadata names for the vectorizer and what the optimizer expects.  What's the protocol for handling this?  Is submitting the optimizer change immediately after this clang change good enough?

In any case, I'd leave changing the vectorize/interleave names to a separate patch, after this one lands.

http://reviews.llvm.org/D4089