Turns out TryReuseSpillForPhi does more than reusing a spill slot for a
phi (which is beneficial in general). It also decides whether a phi
should start out to be allocated in a spill slot. The latter, of course,
benefits from control flow knowledge. Hence, this change was detrimental
in cases where a common input to a phi is only spilled on few control
flow pathes.

To fix, we need to disentangle spill-slot reuse and the decision whether
a phi should start out spilled. I will look into this in a follow up
change.

This reverts commit b79cbd56.

Change-Id: I228185bb1a4b320d3115ba7f1d921593480d8e7d
Reviewed-on: https://chromium-review.googlesource.com/c/1304549Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Stephan Herhut <herhut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57113}