Up until now we were unable to (re)optimize code when we hit
uninitialized (Keyed)Load/StoreICs in the code. We always put an IC
there (sharing the feedback vector with fullcodegen at least) and called
it a day. But we never deoptimized the code object when we gathered more
feedback. This doesn't work very well in practice, esp. with hot code
relying on this. So until we have a proper mechanism to express the need
to reoptimize after we gathered additional feedback from optimized code,
we follow the Crankshaft approach instead and install a SOFT deopt, so
we can not only learn but also utilize the new feedback.

R=mstarzinger@chromium.org
BUG=v8:4470
LOG=n

Review URL: https://codereview.chromium.org/1518013002

Cr-Commit-Position: refs/heads/master@{#34178}