[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