GDScript: Implement pattern guards for match statement

Within a match statement, it is now possible to add guards in each
branch:

	var a = 0
	match a:
		0 when false: print("does not run")
		0 when true: print("but this does")

This allows more complex logic for deciding which branch to take.

modules/gdscript/gdscript_compiler.cpp

@@ -1928,6 +1928,26 @@ Error GDScriptCompiler::_parse_block(CodeGen &codegen, const GDScriptParser::Sui
 						}
 					}
 
+					// If there's a guard, check its condition too.
+					if (branch->guard_body != nullptr) {
+						// Do this first so the guard does not run unless the pattern matched.
+						gen->write_and_left_operand(pattern_result);
+
+						// Don't actually use the block for the guard.
+						// The binds are already in the locals and we don't want to clear the result of the guard condition before we check the actual match.
+						GDScriptCodeGenerator::Address guard_result = _parse_expression(codegen, err, static_cast<GDScriptParser::ExpressionNode *>(branch->guard_body->statements[0]));
+						if (err) {
+							return err;
+						}
+
+						gen->write_and_right_operand(guard_result);
+						gen->write_end_and(pattern_result);
+
+						if (guard_result.mode == GDScriptCodeGenerator::Address::TEMPORARY) {
+							codegen.generator->pop_temporary();
+						}
+					}
+
 					// Check if pattern did match.
 					gen->write_if(pattern_result);