Optimizing: Move `InstructionList` to its own file.

Ideally we would not need to `#include "instruction_list.h"`
in the `nodes.h` but that shall require further refactoring.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Flag: EXEMPT refactor
Change-Id: Iad364ff4c28d9ac9d2238b48da45ad87a65b97f8