glsl: Demote 'type' from ir_instruction to ir_rvalue and ir_variable.

Variables have types, expression trees have types, but statements don't.
Rather than have a nonsensical field that stays NULL in the base class,
just move it to where it makes sense.

Fix up a few places that lazily used ir_instruction even though they
actually knew the particular subclass.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/glsl/ast_function.cpp

@@ -83,7 +83,7 @@ prototype_string(const glsl_type *return_type, const char *name,
 
    const char *comma = "";
    foreach_list(node, parameters) {
-      const ir_instruction *const param = (ir_instruction *) node;
+      const ir_variable *const param = (ir_variable *) node;
 
       ralloc_asprintf_append(&str, "%s%s", comma, param->type->name);
       comma = ", ";