This is the implementation of crrev.com/c/866721 for ia32.

Drive-by change: Clean up the slot index calculation.

At the moment the slot index is encoded in the opcode. This, however,
sets an upper limit the slot index which is lower than what we want to
have (i.e. < 512). With this change we pass the slot index as an
immediate operand, which does not impose limits on the value it
contains.

R=titzer@chromium.org

Change-Id: I40adf8c6e62de28f8428492db6c5297252c1e2d1
Reviewed-on: https://chromium-review.googlesource.com/864642Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50643}