nir: add used field to nir variables

Will be use in a following path by the glsl nir based linker.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25371>

src/compiler/glsl/glsl_to_nir.cpp

@@ -561,6 +561,7 @@ nir_visitor::visit(ir_variable *ir)
    var->data.matrix_layout = ir->data.matrix_layout;
    var->data.from_named_ifc_block = ir->data.from_named_ifc_block;
    var->data.compact = false;
+   var->data.used = ir->data.used;
 
    switch(ir->data.mode) {
    case ir_var_auto:

src/compiler/nir/nir.h

@@ -648,6 +648,15 @@ typedef struct nir_variable {
        */
       unsigned must_be_shader_input : 1;
 
+      /**
+       * Has this variable been used for reading or writing?
+       *
+       * Several GLSL semantic checks require knowledge of whether or not a
+       * variable has been used.  For example, it is an error to redeclare a
+       * variable as invariant after it has been used.
+       */
+      unsigned used:1;
+
       /**
        * How the variable was declared.  See nir_var_declaration_type.
        *