[Bug tree-optimization/70509] wrong code with extract from a v64qi

[zsojka at seznam dot cz]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70509

--- Comment #5 from Zdenek Sojka <zsojka at seznam dot cz> ---
(In reply to Zdenek Sojka from comment #4)
> x86_64 bootstrap finished (the code is likely not triggered), regtest
> running.

It also fixes several wrong-code testcases that I failed to reduce to a
reasonable size, thus were unreported. During testcase reduction, there started
to be out-of-bounds vector indexes, that I fail to detect (with
-fsanitize=undefined).