Optimize the SIMD.js code generation

Two optimizations for discussion:
 1. generate Float32x4Array.getFloat32x4 and Float32x4Array.setFloat32x4 for memory load and store
 2. generate SIMD.type.shuffle and SIMD.type.shuffleMix for __shuffleVector 
I haven't been following the simd spec closely. Is this a new method being added to typed arrays, for the benefit of simd?