gh-90997: Move CACHE handling into _unpack_opargs

[brandtbucher]

_unpack_opargs needs this context to avoid interpreting inline CACHE entries as EXTENDED_ARGs (which happens very easily on big-endian platforms).

[brandtbucher]

No client of _unpack_opargs expects (or wants) CACHE entries... especially populated ones that look like valid instructions. So not only does this fix bugs in several places, but it probably also makes most dis stuff a tiny bit faster.

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @brandtbucher for commit 20a8e09 🤖

If you want to schedule another build, you need to add the ":hammer: test-with-buildbots" label again.

[brandtbucher]

Looks like the first few s390x buildbots are indeed happy with this change.

[pablogsal]

Thanks a lot @brandtbucher ♥️