Purpose :

According to GLSL SPEC 4.6 ( 4.4.1.4 Compute Shader Inputs), for compute shader input qualifiers, we should declare such qualifiers with same values in the same shader (local_size_x, y and z).
"If such a layout qualifier is declared more than once in the same shader, all those declarations must set the same set of local work-group sizes and set them to the same values; otherwise a compile-time error results."

Why this fix:

If we manually set "local_size_x = 1" and directly following a declaration like "local_size_x = 2", this would not be detected. That is because currently we treat all the '1' as default value and could not restrictly detect whether those are default values.

Test case:
......
layout(local_size_x=1) in;
layout(local_size_x=2) in;
......

So I add test cases for this fix:
1. set local_size_y = 1 => success
2. set local_size_y = 2 => error
3. set local_size_y = 1 => success