This is the implementation of crrev.com/c/866721 for mips and mips64.

Drive-by change: I made the slot index calculation on mips the same as
on mips64.

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-mips-ports@googlegroups.com

Change-Id: I46219b07962eadd174f418cba1ea38b07f9b5e96
Reviewed-on: https://chromium-review.googlesource.com/866723
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Ivica Bogosavljevic <ivica.bogosavljevic@mips.com>
Cr-Commit-Position: refs/heads/master@{#50719}