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/ir_validate.cpp

@@ -622,7 +622,9 @@ check_node_type(ir_instruction *ir, void *data)
       printf("Instruction node with unset type\n");
       ir->print(); printf("\n");
    }
-   assert(ir->type != glsl_type::error_type);
+   ir_rvalue *value = ir->as_rvalue();
+   if (value != NULL)
+      assert(value->type != glsl_type::error_type);
 }
 
 void