TableGen: Fix infinite recursion in RegisterBankEmitter

Summary:
AMDGPU has two register classes with the same set of registers, and this
was causing this tablegen backend would get stuck in infinite recursion.

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: tpr, wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D29049

llvm-svn: 293483