[PowerPC] Refactor PPCInstrVSX.td

Over time, we have made many additions to this file and it has frankly become a
bit of a mess. This has led to at least one issue - we have a number of
instructions where the side effects flag should be set to false and we neglected
to do this. This patch suggests a refactoring that should make the file much
more maintainable. The file is split up into major sections and the nesting
level is reduced, predicate blocks merged, etc.

Sections:
  - Custom PPCISD node definitions
  - Predicate definitions
  - Instruction formats
  - Instruction definitions
  - Helper DAG definitions
  - Anonymous patterns
  - Instruction aliases

Differential revision: https://reviews.llvm.org/D78132