[AMDGPU] deduplicate tablegen predicates

We are duplicating predicates if several parts of the combined
predicate list contain the same condition. Added code to deduplicate
the list.

We have AssemblerPredicates and AssemblerPredicate in the
PredicateControl, but we never use AssemblerPredicates with an
actual list, so this one is dropped.

This addresses the first part of the llvm bug 43886:
https://bugs.llvm.org/show_bug.cgi?id=43886

Differential Revision: https://reviews.llvm.org/D69815