[PowerPC] Enable paired vector type and intrinsics when MMA is disabled

This patch enables the Clang type __vector_pair and its associated LLVM
intrinsics even when MMA is disabled. With this patch, the type is now controlled
by the PPC paired-vector-memops option. The builtins and intrinsics will be
renamed to drop the mma prefix in another patch.

Differential Revision: https://reviews.llvm.org/D91819