[wgsl-in] Error on attempting to use mixed operands during compound assignment

[hanawatson]

naga seems not to accept compound assignments that use mixed vector and scalar operands, except in the case of the *= multiplication operator. Other mixed operands (vector+matrix, matrix+scalar) don't produce the same errors for me.

code example:

@compute @workgroup_size(1)
fn compute_main() {
	var var0: vec3<i32> = vec3<i32>();
	var0 += 3;
}

naga compilation error:

[2022-05-11T17:29:42Z ERROR naga::valid::expression] Left: Load { pointer: [2] } of type Vector { size: Tri, kind: Sint, width: 4 }
[2022-05-11T17:29:42Z ERROR naga::valid::expression] Right: Constant([3]) of type Scalar { kind: Sint, width: 4 }
error: 
  ┌─ test.wgsl:4:7
  │
4 │     var0 += 3;
  │          ^^ naga::Expression [5]

Entry point compute_main at Compute is invalid: 
	Expression [5] is invalid
	Operation Add can't work with [3] and [4]

[teoxoy]

This was fixed by #2049.