Instead, separately compute it during prolog generation via another
pass over the Cfg.

This may slow down translation by ~1%, but it greatly simplifies the
management of this flag/property.

The higher motivation is to pull this management out of register
allocation to make it easier to extend register allocation for other
uses.

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/692633004