A label represents a position in the bytecode of a method. Labels are used
for jump, goto, and switch instructions, and for try catch blocks. A label
designates the instruction that is just after. Note however that there
can be other elements between a label and the instruction it designates (such
as other labels, stack map frames, line numbers, etc.).
Returns the offset corresponding to this label. This offset is computed
from the start of the method's bytecode. This method is intended for
Attribute sub classes, and is normally not needed by class
generators or adapters.