Vacuum unused values

[kripken]

Checks if a value is being dropped higher up, like

(drop
  (block i32
    (block i32
      (i32.const 1)
    )
  )
)

Handling this forces us to be careful in that pass about whether a value is used, and whether the type matters (for example, we can't replace a unary with its child in all cases, if the return value matters).

[dschuff]

Can we call this previous or prev instead of last?

[kripken]

Changed to prev.