Make sure not to strip endfilter (#86374)

Fixes the issue described in #86304 (but doesn't fix the issue because that's for ILLink). Found by Pri0 tests.

Author: MichalStrehovsky

src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/FeatureSwitchManager.cs

@@ -289,8 +289,22 @@ public MethodIL GetMethodILWithInlinedSubstitutions(MethodIL method)
                         if (delta >= 0 && delta < ehRegion.TryLength)
                         {
                             if (ehRegion.Kind == ILExceptionRegionKind.Filter)
+                            {
                                 offsetsToVisit.Push(ehRegion.FilterOffset);
 
+                                // Filter must end with endfilter, so ensure we don't accidentally remove it.
+                                // ECMA-335 dictates that filter block starts at FilterOffset and ends at HandlerOffset.
+                                int expectedEndfilterLocation = ehRegion.HandlerOffset - 2;
+                                bool isValidFilter = expectedEndfilterLocation >= 0
+                                    && (flags[expectedEndfilterLocation] & OpcodeFlags.InstructionStart) != 0
+                                    && methodBytes[expectedEndfilterLocation] == (byte)ILOpcode.prefix1
+                                    && methodBytes[expectedEndfilterLocation + 1] == unchecked((byte)ILOpcode.endfilter);
+                                if (isValidFilter)
+                                {
+                                    flags[expectedEndfilterLocation] |= OpcodeFlags.VisibleBasicBlockStart | OpcodeFlags.Mark;
+                                }
+                            }
+
                             offsetsToVisit.Push(ehRegion.HandlerOffset);
 
                             // RyuJIT is going to look at this basic block even though it's unreachable.