[AMDGPU][MC][NFC] Refactored custom operands handling

The original design of custom operands support assumed that most GPUs
have the same or very similar operand names end encodings. This is
no longer the case. As a result the support code becomes over-complicated
and difficult to maintain.

This change implements a different design with the following benefits:

- support of aliases;
- support of operands with overlapped encodings;
- identification of defined but unsupported operands.

Differential Revision: https://reviews.llvm.org/D121696