Special case some bytecode ops in getStackValue
Some bytecode ops, such as CGetL3, would munch the
types of the things on the stack above the new cell that was
pushed because getStackValue assumes all stack outputs from
interpOne are on top of the stack. The result is that the top
of the stack would contain the output type, and the two things
below it would not get any type (unknown).
This usually isn't a problem, but if we fail enough in type
prediction we might get around to InterpOneing these ops.