[clang] [LoongArch][Clang] Make the parameter and return value of {x,}vmsknz.b builtins `signed char` vector (PR #114511)

[Lu Weining via cfe-commits]

https://github.com/SixWeining commented:

Is it the time to remove the FIXME in those tests?

https://github.com/llvm/llvm-project/pull/114511