Expand getCode caching logic for SELFDESTRUCT via DELEGATECALL

[iamdefinitelyahuman]

What I did

Expand the logic when determining if bytecode may be cached. In particular, consider an indirect SELFDESTRUCT via DELEGATECALL.

Thanks to @banteg for pointing this one out. I am a bad devloper and I'm not sure why any of you use my code.

How I did it

The new implementation is fairly well commented, but the gist of it is:

• remove the bytecode after PUSH instructions to avoid false positives
• scan for DELEGATECALL and if it isn't preceded by PUSH20 [address] GAS assume it to be dynamic and thus not cacheable
• where an address is hardcoded, also check if the target address is cachable

This isn't perfect but it should err on the side of caution.

How to verify it

I actually wrote tests on this! Good job, me.