glsl: Eliminate ambiguity between function ins/outs and shader ins/outs

This patch replaces the three ir_variable_mode enums:

- ir_var_in
- ir_var_out
- ir_var_inout

with the following five:

- ir_var_shader_in
- ir_var_shader_out
- ir_var_function_in
- ir_var_function_out
- ir_var_function_inout

This eliminates a frustrating ambiguity: it used to be impossible to
tell whether an ir_var_{in,out} variable was a shader in/out or a
function in/out without seeing where the variable was declared in the
IR.  This complicated some optimization and lowering passes, and would
have become a problem for implementing varying structs.

In the lisp-style serialization of GLSL IR to strings performed by
ir_print_visitor.cpp and ir_reader.cpp, I've retained the names "in",
"out", and "inout" for function parameters, to avoid introducing code
churn to the src/glsl/builtins/ir/ directory.

Note: a couple of comments in the code seemed to indicate that we were
planning for a possible future in which geometry shaders could have
shader-scope inout variables.  Our GLSL grammar rejects shader-scope
inout variables, and I've been unable to find any evidence in the GLSL
standards documents (or extensions) that this will ever be allowed, so
I've eliminated these comments.

Reviewed-by: Carl Worth <cworth@cworth.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>

src/mesa/program/ir_to_mesa.cpp

@@ -1529,21 +1529,18 @@ ir_to_mesa_visitor::visit(ir_dereference_variable *ir)
 					       var->location);
 	 this->variables.push_tail(entry);
 	 break;
-      case ir_var_in:
-      case ir_var_inout:
+      case ir_var_shader_in:
 	 /* The linker assigns locations for varyings and attributes,
 	  * including deprecated builtins (like gl_Color),
 	  * user-assigned generic attributes (glBindVertexLocation),
 	  * and user-defined varyings.
-	  *
-	  * FINISHME: We would hit this path for function arguments.  Fix!
 	  */
 	 assert(var->location != -1);
          entry = new(mem_ctx) variable_storage(var,
                                                PROGRAM_INPUT,
                                                var->location);
          break;
-      case ir_var_out:
+      case ir_var_shader_out:
 	 assert(var->location != -1);
          entry = new(mem_ctx) variable_storage(var,
                                                PROGRAM_OUTPUT,