Fix fgRemoveUnreachableBlocks to detect all changes

When 'removing' a BBF_DONT_REMOVE block we change it to BBJ_THROW. After
this it is possible that other blocks become unreachable, so we should
keep looking for such blocks.

In dotnet#57061 that manifested in a case where the unreachable block did not
have SSA built for it, but downstream the compiler was relying on SSA
being built.

Fix dotnet#57061

src/coreclr/jit/fgopt.cpp

@@ -359,7 +359,7 @@ void Compiler::fgComputeEnterBlocksSet()
 // are never considered unreachable.
 //
 // Return Value:
-//    Return true if any unreachable blocks were removed.
+//    Return true if changes were made that may cause additional blocks to be removable.
 //
 // Assumptions:
 //    The reachability sets must be computed and valid.
@@ -376,6 +376,7 @@ bool Compiler::fgRemoveUnreachableBlocks()
 
     bool hasLoops             = false;
     bool hasUnreachableBlocks = false;
+    bool changed              = false;
 
     /* Record unreachable blocks */
     for (BasicBlock* const block : Blocks())
@@ -419,6 +420,9 @@ bool Compiler::fgRemoveUnreachableBlocks()
             /* Unmark the block as removed, */
             /* clear BBF_INTERNAL as well and set BBJ_IMPORTED */
 
+            // The successors may be unreachable after this change.
+            changed |= block->NumSucc() > 0;
+
             block->bbFlags &= ~(BBF_REMOVED | BBF_INTERNAL);
             block->bbFlags |= BBF_IMPORTED;
             block->bbJumpKind = BBJ_THROW;
@@ -438,6 +442,7 @@ bool Compiler::fgRemoveUnreachableBlocks()
         {
             /* We have to call fgRemoveBlock next */
             hasUnreachableBlocks = true;
+            changed = true;
         }
         continue;
 
@@ -498,7 +503,7 @@ bool Compiler::fgRemoveUnreachableBlocks()
         }
     }
 
-    return hasUnreachableBlocks;
+    return changed;
 }
 
 //------------------------------------------------------------------------

src/tests/JIT/Regression/JitBlue/Runtime_57061/Runtime_57061.cs

@@ -0,0 +1,102 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+// Generated by Fuzzlyn v1.2 on 2021-08-07 03:24:16
+// Run on .NET 6.0.0-dev on Arm Linux
+// Seed: 12756399625466979010
+// Reduced from 798.7 KiB to 1.5 KiB in 03:42:20
+// Crashes the runtime
+using System.Runtime.CompilerServices;
+
+struct S0
+{
+    public bool F0;
+    public bool F1;
+    public uint F2;
+    public short F3;
+    public ulong F4;
+    public S0(bool f0, bool f1, uint f2, short f3, ulong f4): this()
+    {
+        F0 = f0;
+        F1 = f1;
+        F2 = f2;
+        F3 = f3;
+        F4 = f4;
+    }
+}
+
+class C0
+{
+    public C0(S0 f7, S0 f8)
+    {
+    }
+}
+
+class C1
+{
+    public S0 F1;
+    public ulong F5;
+}
+
+struct S2
+{
+    public S2(C0 f4): this()
+    {
+    }
+}
+
+struct S3
+{
+    public uint F0;
+}
+
+class C2
+{
+    public C1 F3;
+}
+
+public class Program
+{
+    static C2 s_23;
+    static C1 s_37;
+    static sbyte s_56;
+    static S3 s_60;
+    public static void Main()
+    {
+        uint vr2 = default(uint);
+        uint vr3;
+        bool vr4 = true;
+        if (!vr4)
+        {
+            try
+            {
+                System.Console.WriteLine(s_60.F0);
+            }
+            finally
+            {
+                var vr5 = new C0(new S0(false, true, 0, 0, 0), new S0(false, false, 0, 0, 0));
+            }
+
+            s_37.F5 = s_23.F3.F1.F4++;
+        }
+
+        vr4 = vr4;
+        for (int vr6 = 0; vr6 < Bound(); vr6++)
+        {
+            sbyte vr8 = s_56;
+            try
+            {
+                var vr7 = new S2(new C0(new S0(true, false, 0, 0, 0), new S0(true, true, 0, 0, 0)));
+            }
+            finally
+            {
+                vr3 = vr2;
+            }
+
+            vr3 = vr3;
+        }
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static int Bound() => 0;
+}

src/tests/JIT/Regression/JitBlue/Runtime_57061/Runtime_57061.csproj

@@ -0,0 +1,12 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+  </PropertyGroup>
+  <PropertyGroup>
+    <DebugType>None</DebugType>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>