Revert of MIPS64: Enable shorten-64-to-32 warning. (patchset #12 id:240001 of https://codereview.chromium.org/1133163005/)

Reason for revert:
Compile failure: https://chromegw.corp.google.com/i/client.v8/builders/V8%20Mac64%20-%20debug/builds/3070/steps/compile/logs/stdio

Original issue's description:
> MIPS64: Enable shorten-64-to-32 warning.
>
> Port cdc43bc5
>
> Original commit message:
> Enable clang's shorten-64-to-32 warning flag on ARM64, and fix the warnings
> that arise.
>
> BUG=
>
> Committed: https://crrev.com/9af578a7c83b58a0ce25345998d9287cbf2030cb
> Cr-Commit-Position: refs/heads/master@{#28990}

TBR=danno@chromium.org,martyn.capewell@arm.com,paul.lind@imgtec.com,akos.palfi@imgtec.com,dusan.milosavljevic@imgtec.com,jkummerow@chromium.org,machenbach@chromium.org,svenpanne@chromium.org,balazs.kilvady@imgtec.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/1182493007

Cr-Commit-Position: refs/heads/master@{#28991}