[C++11] Replace some comparisons with 'nullptr' with simple boolean checks to reduce verbosity.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205829 91177308-0d34-0410-b5e6-96231b3b80d8
[C++11] Make use of 'nullptr' in TableGen library.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205830 91177308-0d34-0410-b5e6-96231b3b80d8