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

Review URL: https://codereview.chromium.org/143903016

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18737 ce2b1a6d-e550-0410-aec6-3dcde31c8c00