glsl: Add a predicate to determine whether a variable is an interface block

For the first declaration below, there will be an ir_variable named
"instance" whose type and whose instance_type will be the same
glsl_type.  For the second declaration, there will be an ir_variable
named "f" whose type is float and whose instance_type is B2.

"instance" is an interface instance variable, but "f" is not.

uniform B1 {
    float f;
} instance;

uniform B2 {
    float f;
};

v2: Copy the comment message documentation into the code.  Suggested by
Paul Berry.

Signed-off-by: Ian Romanick <[email protected]>
Reviewed-by: Carl Worth <[email protected]>
Reviewed-by: Paul Berry <[email protected]>
Reviewed-by: Kenneth Graunke <[email protected]>

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