[TI removal] Remove `TerminatorInst` from GVN.h and GVN.cpp.

This is the last interesting usage in all of LLVM's headers. The
remaining usages in headers are the core typesystem bits (Core.h,
instruction types, and InstVisitor) and as the return of
`BasicBlock::getTerminator`. The latter is the big remaining API point
that I'll remove after mass updates to user code.

llvm-svn: 344501