glsl: Throw the required error when a case label is a non-constant.

It's not quite spelled out in the spec text, but the grammar indicates
that only constant values are allowed as switch() case labels (and
only constant values make sense, anyway).

Fixes piglit glsl-1.30/compiler/switch-statement/switch-case-uniform-int.vert.

NOTE: This is a candidate for the 8.0 branch.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/glsl/ast_to_hir.cpp

@@ -3697,14 +3697,26 @@ ast_switch_statement::hir(exec_list *instructions,
 	/* Conditionally set fallthru state based on
 	 * comparison of cached test expression value to case label.
 	 */
-	ir_rvalue *const test_val = this->test_value->hir(instructions, state);
+	ir_rvalue *const label_rval = this->test_value->hir(instructions, state);
+	ir_constant *label_const = label_rval->constant_expression_value();
+
+	if (!label_const) {
+	   YYLTYPE loc = this->test_value->get_location();
+
+	   _mesa_glsl_error(& loc, state,
+			    "switch statement case label must be a "
+			    "constant expression");
+
+	   /* Stuff a dummy value in to allow processing to continue. */
+	   label_const = new(ctx) ir_constant(0);
+	}
 
 	ir_dereference_variable *deref_test_var =
 	   new(ctx) ir_dereference_variable(state->switch_state.test_var);
 
 	ir_rvalue *const test_cond = new(ctx) ir_expression(ir_binop_all_equal,
 							    glsl_type::bool_type,
-							    test_val,
+							    label_const,
 							    deref_test_var);
 
 	ir_assignment *set_fallthru_on_test =