Wasm2JS missed optimization opportunity on memset like repeated heap assignments

Building https://github.com/juj/wasm_webgpu/commit/4c2600b0f6549f95fe9251cba629c935fbc74ba8 clear_screen sample for testing WebGPU on Wasm2JS, I notice the following kind of statements appear:

```js
HEAP32[$1 + 40 >> 2] = 0;
HEAP32[$1 + 44 >> 2] = 0;
HEAP32[$1 + 32 >> 2] = 0;
HEAP32[$1 + 36 >> 2] = 0;
HEAP32[$1 + 24 >> 2] = 0;
HEAP32[$1 + 28 >> 2] = 0;
HEAP32[$1 + 16 >> 2] = 0;
HEAP32[$1 + 20 >> 2] = 0;
HEAP32[$1 + 8 >> 2] = 0;
HEAP32[$1 + 12 >> 2] = 0;
```

These look like unrolled `-O3` memset code. That code could be simplified as
```js
HEAP32[$1 + 40 >> 2] = HEAP32[$1 + 44 >> 2] = HEAP32[$1 + 32 >> 2] = HEAP32[$1 + 36 >> 2] = HEAP32[$1 + 24 >> 2] = HEAP32[$1 + 28 >> 2] = HEAP32[$1 + 16 >> 2] = HEAP32[$1 + 20 >> 2] = HEAP32[$1 + 8 >> 2] = HEAP32[$1 + 12 >> 2] = 0;
```