Fix unsigned comparisons.

Instead of marking the comparison instruction itself as Uint32, we
look at its arguments. This is more consistent what HChange does.

BUG=v8:3380
TEST=mjsunit/regress/regress-3380
LOG=y
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/325133004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21762 ce2b1a6d-e550-0410-aec6-3dcde31c8c00