Separated the new register allocator in its own files.

Separated the new register allocator in its own files.

Opportunistically removed GreedyAllocator::TryReuseSpillForPhi because it is actually unsuitable for Greedy. It was copied from Linear and it relies on hints, however, the current implementation of hints assumes linear scan.

This change doesn't aim to address performance nor correctness for Greedy.

BUG=

Review URL: https://codereview.chromium.org/1184183002

Cr-Commit-Position: refs/heads/master@{#29054}