[field type tracking] Fix handling of cleared WeakCells

[field type tracking] Fix handling of cleared WeakCells

Whenever a generalization is computed, the inputs must be checked for being cleared, and if they are, the generalization must be Type::Any.

Hopefully this fixes Chromium issue 527994 as well.

BUG=v8:4325,chromium:527994
LOG=n

Review URL: https://codereview.chromium.org/1361103002

Cr-Commit-Position: refs/heads/master@{#30887}