What are the figures of speech in "Ozymandias?"

Although "figures of speech" can be considered broadly to include both poetic (sound) techniques as well as non-literal language, this answer is restricted to figures of speech that are non-literal language.

There are two instances of synecdoche in the poem. With synecdoche, a writer uses a part of the thing to represent the whole. Thus "the hand that mocked them, and the heart that fed" uses two parts of the king, his hand and his heart, to represent him. 

The use of the word "antique" in line 1 could be considered hyperbole. Hyperbole is an exaggeration for effect. The word "antique" can mean "something belonging to ancient times." In literal terms, a land cannot belong to ancient times. If the land exists in the present, it "belongs" in the present, so to call a land "antique" is an exaggeration, but it conveys the idea that the land was more important in ancient days. 

The idea that the "shattered visage" is able to "tell" about the sculptor is personification, which imparts human characteristics to inanimate objects. The visage is unable to communicate with words, so it literally cannot "tell" anything. The word "tell" is also a pun, or a play on words. Although the statue cannot "tell" the viewer anything, the viewer can "tell" from the statue certain things about the king. This is a different meaning of "tell," meaning to determine, but is suggested by the way in which it is used here.

The word "decay" implies a metaphor. Although structures can be said to "decay" when they deteriorate, decay more often brings to mind the destruction of organic matter by bacteria and other creatures. Just as this structure has toppled, so the king's body has decayed and decomposed long ago. The broken statue is a metaphor, or comparison, to the dead king. 

The word "colossal" might be considered both an allusion and a tautology. Although the word means simply "extremely large," it is derived from "colossus," which means a giant statue and was first used by Herodotus to describe the statues in ancient Egyptian temples. Thus it can be an allusion, a reference to a historical or mythical person or event. It also calls to mind the Rhodes Colossus, one of the seven wonders of the ancient world that was destroyed in an earthquake. So to call the wrecked statue a "colossal Wreck" could be like saying it is a "wrecked statue Wreck," making it a tautology, a way of saying the same thing with different words.

Although not captured in any specific line, the entire poem has an air of understatement and irony. The fact that no judgment is stated about the described scene but that it is left to speak for itself is understatement. Understatement deliberately makes something less important than it is. This scene is obviously quite impressive and meaningful, yet the meaning is not overtly stated. Irony represents a turn of events that is the opposite of what is expected. In this poem, the powerful king inspired fear in his day, but now his "Works" are nowhere to be seen. 

These figures of speech, examples of non-literal language, in "Ozymandias" give it a very rich meaning.