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

Drive-by change: I simplified the slot index calculation.

Original description:

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=v8-arm-ports@googlegroups.com

Change-Id: Ic448872aa1da63f421d569ab5ec9160f36e6652b
Reviewed-on: https://chromium-review.googlesource.com/866745Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50607}