RewriteStatepointsForGC::PhiState: Remove explicit copy ctor in favor of the Rule of Zero

The assertion was just checking a class invariant that's pretty easy to
verify by inspection (no mutating operations, and the two non-copy ctors
already ensure the state is maintained) so remove the explicit copy ctor
in favor of the default, thus allowing the use of the default copy
assignment operator without hitting the C++11 deprecation here.

llvm-svn: 231143