[AIX] Remove unused vector registers from allocation order in the default AltiVec ABI

The previous implementation of the default AltiVec ABI marked registers V20-V31
as reserved.  This failed to prevent reserved VFRC registers being allocated.
In this patch instead of marking the registers reserved we remove unallowed
registers from the allocation order completely.

This is a slight rework of an implementation by @nemanjai

Reviewed By: jsji

Differential Revision: https://reviews.llvm.org/D100050