Debugging helper. Centralized the logic for printing blocks from
InstructionSequence.

A clean(-er) design would be to define an operator<< on a
PrintableInstructionBlock. However, we've discussed moving off those
operators, so it seemed unnecessary to complicate the change.

BUG=

Review URL: https://codereview.chromium.org/1632803003

Cr-Commit-Position: refs/heads/master@{#33505}