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: Iab676186f41b8174bcc6c5a6053e6b0d5640ed3c
Reviewed-on: https://chromium-review.googlesource.com/866721
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50586}