We resurrect the VectorSlotPair in order to be able to separate the
feedback input for the compiler from the actual type feedback vector
that is required to meet the IC requirements at runtime. This will allow
us to for example use feedback from a different context or divide the
type feedback vector into two separate vectors, without having to touch
the compiler. It'll allow use to load the vector from the shared
function info at runtime, while still consuming feedback in the
compiler (i.e. we don't rely on the feedback vector node to be a heap
constant).

R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/1198983002

Cr-Commit-Position: refs/heads/master@{#29185}