[Bug target/123839] riscv: Optimize constant permute to slidedown + slideup.

[raphael.zinsly at oss dot qualcomm.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123839

--- Comment #1 from Raphael M Zinsly <raphael.zinsly at oss dot qualcomm.com> ---
I'll take a look at this