Force-bake array values in MSL

[kvark]

Input WGSL

[[block]] struct BufferCoords { coords: array<vec2<f32>>; };
[[group(0), binding(0)]] var<storage, read> bufferCoords : BufferCoords;

var<private> coords : array<vec2<f32>, 3> = array<vec2<f32>, 3>(
	vec2<f32>(-1.0, -1.0),
	vec2<f32>( 1.0, -1.0),
	vec2<f32>( 0.0,  1.0)
);

struct VertInput { [[builtin(vertex_index)]] index : u32; };
struct VertOutput { [[builtin(position)]] Position : vec4<f32>; };
struct FragOutput { [[location(0)]] color : vec4<f32>; };

[[stage(vertex)]] fn vertMain(input : VertInput) -> VertOutput {
	var coord = bufferCoords.coords[0];

	 var coords : array<vec2<f32>, 3> = array<vec2<f32>, 3>(
	 	vec2<f32>(-1.0, -1.0),
	 	vec2<f32>( 1.0, -1.0),
	 	vec2<f32>( 0.0,  1.0)
	 );

	return VertOutput(vec4<f32>(coords[i32(input.index)], 0.0, 1.0));
}

[[stage(fragment)]] fn fragMain() -> FragOutput {
	return FragOutput(vec4<f32>(1.0));
}

Translates to Metal with this piece:

    type_2 coords;
    metal::float2 _e6 = bufferCoords.coords[0];
    coord = _e6;
    for(int _i=0; _i<3; ++_i) coords.inner[_i] = type_2 {metal::float2(-1.0, -1.0), metal::float2(1.0, -1.0), metal::float2(0.0, 1.0)}.inner[_i];

It's unfortunate because it creates the array inside the copy loop.
We should at least force it to bake that value first.

[kvark]

We should do it after #1532 since there is going to be infrastructure to forcing the baking of more expressions.

[teoxoy]

This is no longer necessary due to #764 and #2331.