JIT: clear BBF_DONT_REMOVE on the new preheader in loop inversion

src/coreclr/jit/optimizer.cpp

@@ -2142,6 +2142,11 @@ bool Compiler::optTryInvertWhileLoop(FlowGraphNaturalLoop* loop)
     // block will be the new preheader.
     BasicBlock* const newPreheader = fgSplitBlockAtEnd(preheader);
 
+    // 'preheader' stays the entry of any EH region it begins, so the new block does not
+    // need the BBF_DONT_REMOVE it inherited; keeping it would leave an unremovable block
+    // behind if the loop later becomes unreachable.
+    newPreheader->RemoveFlags(BBF_DONT_REMOVE);
+
     // Make sure exit stays canonical
     BasicBlock* nonEnterBlock = fgSplitBlockAtBeginning(exit);
 

src/tests/JIT/Regression_ro_2/Runtime_136041.cs

@@ -0,0 +1,44 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Runtime.CompilerServices;
+using Xunit;
+
+public class Runtime_136041
+{
+    [Fact]
+    public static void TestEntryPoint()
+    {
+        Assert.Equal(0, Test(new int[] { 1 }, 0));
+        Assert.Equal(1, Test(new int[] { 1 }, 1));
+    }
+
+    // Loop inversion splits the catch handler entry to create the loop preheader,
+    // then the loop condition folds to false, leaving an unreachable block that
+    // inherited the handler entry's "don't remove" flag.
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static int Test(int[] p, int d)
+    {
+        int result = 0;
+        try
+        {
+            result = 1 / d;
+            GC.KeepAlive(p);
+        }
+        catch (DivideByZeroException)
+        {
+            while (checked(p[0] > p[0]))
+            {
+                try
+                {
+                    GC.Collect();
+                }
+                catch (Exception) { }
+            }
+        }
+        catch (Exception) { }
+
+        return result;
+    }
+}