Clean up CDS handling of i2i_entry

This is a prerequisite for JDK-8250989 - Consolidate buffer allocation code for CDS static/dynamic archive dumping.

[1] Move the allocation of the i2i buffer after all classes are loaded. This makes it possible to estimate the size of the CDS archive before we allocate the output space (this is what dynamic archive does now).
[2] No need to generate the i2i trampoline code during -Xshare:dump
[3] Clean up the CDS code in abstractInterpreter.cpp and add more comments.