MSL: Ensure OpStore source operands are marked for inclusion in function arguments
MSL: Ensure OpStore source operands are marked for inclusion in function arguments

Without this change, code such as:

```
OpStore %param_var_mipLevelSizes_0 %heightmapMipSizes
```

within a function that then forwards the value `%param_var_mipLevelSizes_0` to another function will not have `%heightmapMipSizes` registered as an argument to the function.
KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross