[turbofan] reuse spill slots for phis

R=jarin@chromium.org

BUG=

Review URL: https://codereview.chromium.org/777403003

Cr-Commit-Position: refs/heads/master@{#25704}
Fix typo in r25689 (Refactor Map::ConstructionCount.)

BUG=chromium:439868
LOG=N

Review URL: https://codereview.chromium.org/779293003

Cr-Commit-Position: refs/heads/master@{#25705}