PR #66334 tried to renumber slot indexes before register allocation, but
the numbering was still affected by list entries for instructions which
had been erased. Fix this to make the register allocator's live range
length heuristics even less dependent on the history of how instructions
have been added to and removed from SlotIndexes's maps.
Normally, generic processor does not have any SubtargetFeature. And it
can just generate most basic instructions which have no Predicates to
guard.
But it needs to enbale predicate for the btsti16 instruction as one of the most basic instructions.
Or the generic processor can't finish codegen process. So Add FeatureBTST16 SubtargetFeature to generic ProcessorModel.
For now, just support atomic operations by libcall. Further, should investigate atomic
implementation in CSKY target and codegen with atomic and fence related instructions.
