TableGen: Simplify BitsInit::resolveReferences

Summary:
No functional change intended. The removed code has a loop for
recursive resolving, which is superseded by the recursive
resolving done by the Resolver implementations.

Add a test case which was broken by an earlier version of this
change.

Change-Id: Ib208d037b77a8bbb725977f1388601fc984723d8

Reviewers: arsenm, craig.topper, tra, MartinO

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D43655

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@326784 91177308-0d34-0410-b5e6-96231b3b80d8