Investigate SIMD opportunities

Some parts of UASTC transcoders have lots of similar operations, so they likely could be implemented more efficiently via SIMD instructions.

References:
- [WebAssembly SIMD spec proposal](https://github.com/WebAssembly/simd/blob/master/proposals/simd/SIMD.md)
- [Intrinsics in AssemblyScript](https://www.assemblyscript.org/environment.html#low-level-webassembly-operations)