[PowerPC] Add Vector Extract/Expand/Count with Mask, Move to VSR Mask...

[PowerPC] Add Vector Extract/Expand/Count with Mask, Move to VSR Mask Instruction Definitions and MC Tests

This patch adds the instruction definitions and assembly/disassembly tests for
the following set of instructions:

Vector Extract [byte | half | word | doubleword | quad] with mask
Vector Expand [byte | half | word | doubleword | quad] with mask
Move to VSR [byte | byte immediate | half | word | doubleword | quad] with mask
Vector Count Mask Bits [byte | half | word | doubleword]

Differential Revision: https://reviews.llvm.org/D83724