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

[Xi Ruoyao via cfe-commits]

xry111 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.

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