[AIX] Refactor AIX Call Lowering to use CCState. NFCI.

This patch reworks the AIX call lowering to use CCState. Some defensive errors
are added in this patch to protect from emitting bad code for calling convention
logic that has not been implemented by design. The use of CCState follows the
precedent of other targets and enables the reuse of calling convention logic in
LowerFormalArguments, which will be rewritten to also use CCState in a late
patch.

Patch by Chris Bowler.

Differential Revision: https://reviews.llvm.org/D69101