Fix for #6903 - enable vectorization of loops with non-constant lower bound.

This improvements enables vectorization of the #6903.  It also improves performance of some of my other ```@simd``` benchmarks. 

I timed 5 different variations of loop lowering, including the normal one that Julia uses, and this is one of three variations that came out the fastest (within measurement noise) for aforementioned benchmarks, and did significantly better than using Julia's normal loop-lowering scheme.

When we get to LLVM 3.5, let's try using Julia's usual loop lowering.  LLVM 3.4's vectorizer seemed to do much better than LLVM 3.3 with the usual lowering.