TableGen: Generalize type deduction for !listconcat

Summary:
This way, it should work even with complex operands.

Change-Id: Iaccf5bbb50bd5882a0ba5d59689e4381315fb361

Reviewers: arsenm, craig.topper, tra, MartinO

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D43554

llvm-svn: 325796