[PowerPC] Rename the vector pair intrinsics and builtins to replace the _mma_ prefix by _vsx_

On PPC, the vector pair instructions are independent from MMA.
This patch renames the vector pair LLVM intrinsics and Clang builtins to replace the _mma_ prefix by _vsx_ in their names.
We also move the vector pair type/intrinsic/builtin tests to their own files.

Differential Revision: https://reviews.llvm.org/D91974