fix: speculative execution in wasm

In https://github.com/nodejs/llparse/pull/74 we make sure to increment position by a constant whenever in a happy path so that the CPU could proceed through the loop header (`endp - p >= 16`) speculatively instead of blocking and waiting for vector to scalar to fully resolve. This doesn't seem to give an obvious performance boost yet, but potentially might have an effect with the future V8 updates. cc @mcollina @nodejs/llhttp