TableGen: BitInit and VarBitInit are typed

Summary: Change-Id: I54e337a0b525e9649534bc5f90e5e07c0772e334

Reviewers: arsenm, craig.topper, tra, MartinO

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D43560

Change-Id: I07f78e793192974c2b90690ce644589fe4891e41
llvm-svn: 325885