Fixed floor-of-div optimization.

Fixed floor-of-div optimization.

We removed an HDiv by hand which was still used by an HChange. The
solution is letting dead code removal do the cleanup.

Removed a fragile "optimization" (looking through an HChange), too,
this obviously never triggered and is hard to get right given all our
global invariants and state/type/... changes.

The repro is a bit tricky, because you need inlining to make our
representations and types disagree in this case.

LOG=y
BUG=334708
R=bmeurer@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=18737

[Benedikt Meurer, 2014-01-22 11:38:18]

LGTM

[Sven Panne, 2014-01-22 11:54:59]

Committed patchset #1 manually as r18737 (presubmit successful).