Use a SetTheory instance to expand register lists in register classes.

Use a SetTheory instance to expand register lists in register classes.

This prepares tablegen to compute register lists from set theoretic dag
expressions. This doesn't really make any difference as long as
Target.td still declares RegisterClass::MemberList as [Register].

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@133043 91177308-0d34-0410-b5e6-96231b3b80d8