Incoming -- add support for SIMD x86 intrinsics

getting some initial set of x86 intrinsics mapped to JS SIMD ops

update existing SIMD code to use updated JS SIMD API (SIMD.type.op instead of SIMD.op & SIMD.opu32; int32x4 instead of uint32x4); 

bug fix: right type of withX() for int32x4 initialization; 
bug fix: splat(0) instead of zero() for int32x4 initialization