[X86] Move the memory unfolding table creation into its own class and make it a ManagedStatic.

Also move the static folding tables, their search functions and the new class into new cpp/h files.

The unfolding table is effectively static data. It's just a different ordering and a subset of the static folding tables.

By putting it in a separate ManagedStatic we ensure we only have one copy instead of one per X86InstrInfo object. This way also makes it only get initialized when really needed.

llvm-svn: 336056