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 
This is a proposal to resolve loading SIMD types from array buffer without 16-bytes alignment. See https://github.com/johnmccutchan/ecmascript_simd/issues/52 for more details. It is driven by real use cases I am working on.