[PPC64] Added vector registers.

The Registers_ppc64 class needed a couple of changes, both to accommodate the
new registers as well as to handle the overlaps of VS register set
without wasting space.

The save/restore code of V and VS registers was added.
As VS registers depend on the VMX extension, they are processed only if
VMX support is detected (_ARCH_PWR8 for now).

Patch by Leandro Lupori!

Differential Revision: https://reviews.llvm.org/D41906

llvm-svn: 322596