[Bug target/97194] optimize vector element set/extract at variable position

[crazylht at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97194

--- Comment #23 from Hongtao.liu <crazylht at gmail dot com> ---
Fixed in GCC11, may need a bit adjustment for the modeless operand(the variable
index) as dicussed in
https://gcc.gnu.org/pipermail/gcc-patches/2020-November/559213.html