[C++11] More 'nullptr' conversion. In some cases just using a boolean check instead of comparing to nullptr.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206142 91177308-0d34-0410-b5e6-96231b3b80d8
Add back some typecasts I accidentally lost in r206142.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206143 91177308-0d34-0410-b5e6-96231b3b80d8