Refactor - Remove first_instruction_account in bpf_loader

[Lichtso]

Problem

The logic in process_instruction_common() is quite complex and hard to understand. Additionally, we want to get rid of program_indices and try_borrow_program_account, so their usage in the BPF loader needs to be minimized.

There are five cases of what program_account inputs process_instruction_common() needs to handle:

• Program Management Instruction: [Loader]
• Transaction Level Instruction: [Loader, Program]
• Transaction Level Instruction: [Loader, Programdata, Program]
• Cross Program Invocation: [Program]
• Cross Program Invocation: [Programdata, Program]

From that we can tell, that we only need to look at the last program in the chain, and only in case of the upgradeable loader also need the second last (the programdata account).

Summary of Changes

• Removes first_instruction_account from process_instruction_common(), process_loader_upgradeable_instruction(), process_loader_instruction() and write_program_data().
• Removes get_index_in_transaction() and try_borrow_account().