Generate SIMD.js shuffle and shuffleMix for LLVM shufflevector instruction.

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 
typo in `whether`