glsl/linker: Allow unused in blocks which are not declated on previous stage

>From Section 4.3.4 (Inputs) of the GLSL 1.50 spec:

    "Only the input variables that are actually read need to be written
     by the previous stage; it is allowed to have superfluous
     declarations of input variables."

Fixes:
    * interstage-multiple-shader-objects.shader_test

v2:
  Update comment in ir.h since the usage of "used" field
  has been extended.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101247
Signed-off-by: Vadym Shovkoplias <vadym.shovkoplias@globallogic.com>
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>

src/compiler/glsl/ir.h

@@ -667,8 +667,8 @@ public:
        * variable has been used.  For example, it is an error to redeclare a
        * variable as invariant after it has been used.
        *
-       * This is only maintained in the ast_to_hir.cpp path, not in
-       * Mesa's fixed function or ARB program paths.
+       * This is maintained in the ast_to_hir.cpp path and during linking,
+       * but not in Mesa's fixed function or ARB program paths.
        */
       unsigned used:1;