cmake: Conditionalize CodeGen's dependency on intrinsics_gen

Custom targets in cmake cannot be exported, and this dependency is only
needed in the combined build to ensure that Intrinsics.gen is created
before compiling CodeGen. In the standalone, all of LLVM is build first.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221391 91177308-0d34-0410-b5e6-96231b3b80d8