[RewriteStatepointsForGC] Remove notion of SafepointBounds [NFC]

The notion of a range of inserted safepoint related code is no longer really applicable.  This survived over from an earlier implementation.  Just saving the inserted gc.statepoint and working from that is far clearer given the current code structure.  Particularly when invokable statepoints get involved.

llvm-svn: 230063