Avoid MonotoneChain self-compare in SimpleMCSweepLineIntersector

This is a hotspot in validity profiling. This commit reduces the runtime
of GEOSisValid from 3.7s to 2.0s for the 5,489 polygons in the
HydroBASINS level 5 dataset, and from 610ms to 360ms for the individual
polygon components of the boundary of Australia in the GADM country
dataset.

NEWS

@@ -2,10 +2,11 @@ Changes in 3.9.0
 2020-xx-xx
 
 - New things:
-  - 
+  -
 
 - Improvements:
   - Stack allocate segments in OverlapUnion (Paul Ramsey)
+  - Improve performance of GEOSisValid (Dan Baston)
 
 
 Changes in 3.8.0

src/geomgraph/index/SimpleMCSweepLineIntersector.cpp

@@ -143,6 +143,14 @@ SimpleMCSweepLineIntersector::processOverlaps(size_t start, size_t end,
         auto& ev1 = events[i];
         if(ev1->isInsert()) {
             MonotoneChain* mc1 = (MonotoneChain*) ev1->getObject();
+
+            if (mc1 == mc0) {
+                // Don't try to compute intersections between a MonotoneChain
+                // and itself. SegmentIntersector::addIntersections will just
+                // ignore them anyway.
+                continue;
+            }
+
             // don't compare edges in same group
             // null group indicates that edges should be compared
             if(ev0->edgeSet == nullptr || (ev0->edgeSet != ev1->edgeSet)) {