Delays Phi lowering until after register allocation.  This lets the Phi assignment order take register allocation into account and avoid creating false dependencies.

All edges that lead to Phi instructions are split, and the new node gets mov instructions in the correct topological order, using available physical registers as needed.

This lowering style is controllable under -O2 using -phi-edge-split (enabled by default).

The result is faster translation time (due to fewer temporaries leading to faster liveness analysis and register allocation) as well as better code quality (due to better register allocation and fewer phi-based assignments).

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/680733002