Add a hook in ClassReader to allow subclasses to get the bytecode offset of visited instructions.

Closes #317982 (closed). I propose to add this hook to address #317982 (closed). Users can override ClassReader, store the last visited offset in a field, and read this field from each visitXxx method in MethodVisitor to get the bytecode offset of each instruction. I checked that this does not impact performance with the AdapterBenchmark.read_asmCurrent JMH benchmark.