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

[Lu Weining via cfe-commits]

SixWeining wrote:

> > Is it the time to remove the FIXME in those tests?
> 
> There are several random inconsistencies and I separated the fixes into multiple PRs as suggested by Xuerui. The FIXME is removed in #114513.

Oh, I missed those PRs.

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