Fix BBJ_EHFILTERRET handling in fgRemoveBlockAsPred

fgRemoveBlockAsPred had special handling to not decrease the bbRefs count
of the filter-handler targeted by BBJ_EHFILTERRET. This is incorrect, as
the filter-handler already has an extra "beginning of handler" extra
ref count. Possibly this code was never invoked before as filters were
not previously removed.

Also, fix the JitDump output of the filter ret block in block dumps.

Author: BruceForstall

src/coreclr/jit/block.cpp

@@ -637,7 +637,7 @@ void BasicBlock::dspJumpKind()
             break;
 
         case BBJ_EHFILTERRET:
-            printf(" (fltret)");
+            printf(" -> " FMT_BB " (fltret)", bbJumpDest->bbNum);
             break;
 
         case BBJ_EHCATCHRET:

src/coreclr/jit/clrjit.natvis

@@ -21,7 +21,7 @@ Documentation for VS debugger format specifiers: https://docs.microsoft.com/en-u
   </Type>
 
   <Type Name="BasicBlock">
-    <DisplayString Condition="bbJumpKind==BBJ_COND || bbJumpKind==BBJ_ALWAYS || bbJumpKind==BBJ_LEAVE || bbJumpKind==BBJ_EHCATCHRET || bbJumpKind==BBJ_CALLFINALLY">BB{bbNum,d}->BB{bbJumpDest->bbNum,d}; {bbJumpKind,en}</DisplayString>
+    <DisplayString Condition="bbJumpKind==BBJ_COND || bbJumpKind==BBJ_ALWAYS || bbJumpKind==BBJ_LEAVE || bbJumpKind==BBJ_EHCATCHRET || bbJumpKind==BBJ_CALLFINALLY || bbJumpKind==BBJ_EHFILTERRET">BB{bbNum,d}->BB{bbJumpDest->bbNum,d}; {bbJumpKind,en}</DisplayString>
     <DisplayString Condition="bbJumpKind==BBJ_SWITCH">BB{bbNum,d}; {bbJumpKind,en}; {bbJumpSwt->bbsCount} cases</DisplayString>
     <DisplayString Condition="bbJumpKind==BBJ_EHFINALLYRET">BB{bbNum,d}; {bbJumpKind,en}; {bbJumpEhf->bbeCount} succs</DisplayString>
     <DisplayString>BB{bbNum,d}; {bbJumpKind,en}</DisplayString>

src/coreclr/jit/fgbasic.cpp

@@ -268,7 +268,7 @@ bool Compiler::fgEnsureFirstBBisScratch()
         }
 
         // The first block has an implicit ref count which we must
-        // remove. Note the ref count could be greater that one, if
+        // remove. Note the ref count could be greater than one, if
         // the first block is not scratch and is targeted by a
         // branch.
         assert(fgFirstBB->bbRefs >= 1);

src/coreclr/jit/fgdiagnostic.cpp

@@ -2077,7 +2077,8 @@ void Compiler::fgTableDispBasicBlock(BasicBlock* block, int ibcColWidth /* = 0 *
                 break;
 
             case BBJ_EHFILTERRET:
-                printf("%*s        (fltret)", maxBlockNumWidth - 2, "");
+                printf("-> " FMT_BB "%*s (fltret)", block->GetJumpDest()->bbNum,
+                       maxBlockNumWidth - max(CountDigits(block->GetJumpDest()->bbNum), 2), "");
                 break;
 
             case BBJ_EHCATCHRET:

src/coreclr/jit/fgflow.cpp

@@ -346,6 +346,7 @@ void Compiler::fgRemoveBlockAsPred(BasicBlock* block)
         case BBJ_CALLFINALLY:
         case BBJ_ALWAYS:
         case BBJ_EHCATCHRET:
+        case BBJ_EHFILTERRET:
             fgRemoveRefPred(block->GetJumpDest(), block);
             break;
 
@@ -358,11 +359,6 @@ void Compiler::fgRemoveBlockAsPred(BasicBlock* block)
             fgRemoveRefPred(block->Next(), block);
             break;
 
-        case BBJ_EHFILTERRET:
-            block->GetJumpDest()->bbRefs++; // To compensate the bbRefs-- inside fgRemoveRefPred
-            fgRemoveRefPred(block->GetJumpDest(), block);
-            break;
-
         case BBJ_EHFINALLYRET:
             for (BasicBlock* const succ : block->EHFinallyRetSuccs())
             {

src/coreclr/jit/fgopt.cpp

@@ -437,6 +437,11 @@ bool Compiler::fgRemoveUnreachableBlocks(CanRemoveBlockBody canRemoveBlock)
             // to properly set the info.compProfilerCallback flag.
             continue;
         }
+        else if ((block->bbFlags & BBF_DONT_REMOVE) && block->isEmpty() && block->KindIs(BBJ_THROW))
+        {
+            // We already converted a non-removable block to a throw; don't bother processing it again.
+            continue;
+        }
         else if (!canRemoveBlock(block))
         {
             continue;
@@ -458,6 +463,8 @@ bool Compiler::fgRemoveUnreachableBlocks(CanRemoveBlockBody canRemoveBlock)
 
             // Unmark the block as removed, clear BBF_INTERNAL, and set BBJ_IMPORTED
 
+            JITDUMP("Converting BBF_DONT_REMOVE block " FMT_BB " to BBJ_THROW\n", block->bbNum);
+
             // The successors may be unreachable after this change.
             changed |= block->NumSucc() > 0;