[PowerPC] Add new DMR register classes to Future CPU.

A new register class as well as a number of related subregisters are being added
to Future CPU. These registers are Dense Math Registers (DMR) and are 1024 bits
long. These regsiters can also be used in consecutive pairs which leads to a
register that is 2048 bits.

This patch also adds 7 new instructions that use these registers. More
instructions will be added in future patches.

Reviewed By: amyk, saghir

Differential Revision: https://reviews.llvm.org/D136366