Already approved but I've addressed the nits and posted the final commit here because I'll need someone to commit it for me.

Rewrote comments in aix-cc-altivec.ll test, fixed the vector diagnostic bug Zarko pointed out and addressed small nits.

I've decided to use custom regs for all gpr inits for float varargs. I discovered that f64 passing in 64-bit gpr was missing a required bitcast to int to handle the initialization. The new logic ensures the bitcast occurs for any custom handling. Custom handling is only expected for gpr inits of float args.

Recent problems have come up generating the GPR initializations for floating point args, which may require some rework. Hold off on review until revision is posted.

Minor tweaks and test upates to come. I'll add an assembly test to validate that we don't need to store to memory to initiailze the two GPRs for f64 in PPC32 mode on newer hardware.

Clang formatted the change and updated the float reg to use LocVT.

The new logic is much clearer, though it's hard to discern that the semantics remain correct because so much logic was redistributed. If a series of smaller commits were possible it would be easier to review.

Use ValVT in CC_AIX rather than LocVT.

One last test for error on AIX nest parameters. I renamed some of the aix tests for consistency.

Add tests for the limitation errors on sret parameters, byval parameters as well as arguments passed in memory. These are limitations that will be addressed in future work.

I'll add some test coverage for some of the limitation errors.

Minor correction on one of the nit comments.

Address nit comments.

Update the TOC, FP and BP offset accessors to assert for AIX. The assertions will be removed with future work that accompanied by appropriate test coverage.

Rebased changes after D63738 and