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/glsl/opt_function_inlining.cpp

@@ -144,9 +144,9 @@ ir_call::generate_inline(ir_instruction *next_ir)
       }
 
       /* Move the actual param into our param variable if it's an 'in' type. */
-      if (parameters[i] && (sig_param->mode == ir_var_in ||
+      if (parameters[i] && (sig_param->mode == ir_var_function_in ||
 			    sig_param->mode == ir_var_const_in ||
-			    sig_param->mode == ir_var_inout)) {
+			    sig_param->mode == ir_var_function_inout)) {
 	 ir_assignment *assign;
 
 	 assign = new(ctx) ir_assignment(new(ctx) ir_dereference_variable(parameters[i]),
@@ -202,8 +202,8 @@ ir_call::generate_inline(ir_instruction *next_ir)
       const ir_variable *const sig_param = (ir_variable *) sig_param_iter.get();
 
       /* Move our param variable into the actual param if it's an 'out' type. */
-      if (parameters[i] && (sig_param->mode == ir_var_out ||
-			    sig_param->mode == ir_var_inout)) {
+      if (parameters[i] && (sig_param->mode == ir_var_function_out ||
+			    sig_param->mode == ir_var_function_inout)) {
 	 ir_assignment *assign;
 
 	 assign = new(ctx) ir_assignment(param->clone(ctx, NULL)->as_rvalue(),