TableGen: Remove redundant loop in ListInit::resolveReferences

Summary:
Recursive lookups are handled by the Resolver, so the loop was purely
a waste of runtime.

Change-Id: I2bd23a68b478aea0bbac1a86ca7635adffa28688

Reviewers: arsenm, craig.topper, tra, MartinO

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D44624

llvm-svn: 328118