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

[raphael.zinsly at oss dot qualcomm.com via Gcc-bugs]

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

--- Comment #2 from Raphael M Zinsly <raphael.zinsly at oss dot qualcomm.com> 
---
Created attachment 63609
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=63609&action=edit
proposed patch