Enable Instcombine combining of vector shuffles

Instcombine is currently very conservative about combining vector
shuffles because it is afraid of pessimizing code translated into
generic form from xmmintrin.h intrinsics. For the current SIMD.js API,
it is more desirable to push JS engine code generators to emit good code
for the shuffles that users are actually doing, and less desirable to
preserve broken-up shuffles in their original form.