Remove this pointer that is converted to bool. In well-defined contexts, the

Remove this pointer that is converted to bool.  In well-defined contexts, the
this pointer is always non-null.  If the this pointer is null, it is undefined
and the compiler may optimize it away by assuming it is non-null.  The null
checks are pushed into the callers.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214471 91177308-0d34-0410-b5e6-96231b3b80d8