[PowerPC] Add MMA builtin decoding and definitions
Add MMA builtin decoding. These builtins use the new PowerPC-specific types vector_pair and vector_quad.
So to avoid pervasive changes, we use custom type descriptors and custom decoding for these builtins.
We also use custom code generation to expand builtin calls with pointers to simpler intrinsic calls with non-pointer types.
Differential Revision: https://reviews.llvm.org/D81748