fix erroneous a11y warning with input type='image' (#3331)

src/compiler/compile/nodes/Element.ts

@@ -451,11 +451,12 @@ export default class Element extends Node {
 			if (this.name === 'input') {
 				const type = attribute_map.get('type');
 				if (type && type.get_static_value() === 'image') {
-					should_have_attribute(
+					const required_attributes = ['alt', 'aria-label', 'aria-labelledby'];
-						this,
+					const has_attribute = required_attributes.some(name => attribute_map.has(name));
-						['alt', 'aria-label', 'aria-labelledby'],
+
-						'input type="image"'
+					if (!has_attribute) {
-					);
+						should_have_attribute(this, required_attributes, 'input type="image"');
+					}
 				}
 			}
 		}

test/validator/samples/a11y-alt-text/input.svelte

@@ -7,3 +7,5 @@
 <object></object>
 
 <input type='image'>
+
+<input type='image' alt='hey'>