AMDGPU: Fix adding extra operands for i128 asm constraints

We don't register i128 as a legal type with addRegisterClass, but it
appears in the list of legal register types. This inconsistency
resulted in the asm constraint lowering trying to use 2 128-bit
registers for these operands. This would leave behind a dead def that
would waste registers.

Regresses GlobalISel tests for i128 load/store, but these aren't very
important right now. Ideally these would not depend on the list of
register types.