Setting up the stage for heuristics that preprocess live ranges before register allocation, and are independent of register allocation - e.g. the deferred blocks heuristic, or the split at call sites heuristic.

Added a separate flag for this, since we intend to enable it for the linear allocator as well. Currently, the option is "on" for greedy, as a point in time to enable its testing (through the greedy allocator bots).

BUG=

Review URL: https://codereview.chromium.org/1256313003

Cr-Commit-Position: refs/heads/master@{#30005}