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/ir_print_visitor.cpp

@@ -146,7 +146,8 @@ void ir_print_visitor::visit(ir_variable *ir)
 
    const char *const cent = (ir->centroid) ? "centroid " : "";
    const char *const inv = (ir->invariant) ? "invariant " : "";
-   const char *const mode[] = { "", "uniform ", "in ", "out ", "inout ",
+   const char *const mode[] = { "", "uniform ", "shader_in ", "shader_out ",
+                                "in ", "out ", "inout ",
 			        "const_in ", "sys ", "temporary " };
    const char *const interp[] = { "", "flat", "noperspective" };