[CMake] Let add_public_tablegen_target() provide intrinsics_gen, too.

I think, in principle, intrinsics_gen may be added explicitly.
That said, it can be added incidentally, since each target already has dependencies to llvm-tblgen.
Almost all source files depend on both CommonTaleGen and intrinsics_gen.

Explicit add_dependencies() have been pruned under lib/Target.

llvm-svn: 195929