Optimize out const qualifier.

Input:
```glsl
out vec4 fragColor;
const float pi = acos(-1.);
const float tau = acos(-1.)*2.;
void main() {
  fragColor = vec4(1.,1.,1.,1.);
}
```
Output:
```glsl
out vec4 f;
const float m=acos(-1.),N=acos(-1.)*2.;
void main(){
    f=vec4(1);
}
```
Expected output:
```glsl
out vec4 f;
float m=acos(-1.),N=acos(-1.)*2.;
void main(){
    f=vec4(1);
}
```