Fix emitSplit logic

src/coreclr/jit/emit.cpp

@@ -3071,8 +3071,21 @@ void emitter::emitSplit(emitLocation*         startLoc,
 
     } // end for loop
 
-    splitIfNecessary();
-    assert(curSize < UW_MAX_FRAGMENT_SIZE_BYTES);
+    if ((igLastCandidate != nullptr) && (curSize == candidateSize))
+    {
+        // TODO: This scenario may be possible (but very unlikely) when using the default fragment size.
+        // Fixing this would likely require padding out the last fragment to not be zero-sized, and doing the split.
+        // For now, assume this only occurs under stress modes, or when using DOTNET_JitSplitFunctionSize.
+        assert(maxSplitSize != UW_MAX_FRAGMENT_SIZE_BYTES);
+        JITDUMP("emitSplit: can't split at last candidate IG%02u because it would create a zero-sized fragment\n",
+                igLastCandidate->igNum);
+    }
+    else
+    {
+        splitIfNecessary();
+    }
+
+    assert((curSize > 0) && (curSize < UW_MAX_FRAGMENT_SIZE_BYTES));
 }
 
 /*****************************************************************************