JIT: fix liveness in throw helper blocks

PR dotnet#123781 changed things so that throw helper blocks are
now created just after lower. Because of this they no longer get
the special liveness update for unreachable blocks.

Add this update in explicitly when the blocks are created.

Fixes dotnet#123929

Author: AndyAyersMS

src/coreclr/jit/compiler.h

@@ -6792,6 +6792,7 @@ class Compiler
     AddCodeDsc* fgGetExcptnTarget(SpecialCodeKind kind, BasicBlock* fromBlock, bool createIfNeeded = false);
     bool fgUseThrowHelperBlocks();
     void fgCreateThrowHelperBlockCode(AddCodeDsc* add);
+    void fgSetThrowHelpBlockLiveness(BasicBlock* block);
     void fgSequenceLocals(Statement* stmt);
 
 private:

src/coreclr/jit/flowgraph.cpp

@@ -3492,6 +3492,10 @@ void Compiler::fgCreateThrowHelperBlock(AddCodeDsc* add)
     //
     add->acdDstBlk = newBlk;
 
+    // Set up liveness
+    //
+    fgSetThrowHelpBlockLiveness(newBlk);
+
 #ifdef DEBUG
     if (verbose)
     {

src/coreclr/jit/liveness.cpp

@@ -1472,22 +1472,7 @@ void Liveness<TLiveness>::DoLiveVarAnalysis()
                 continue;
             }
 
-            VarSetOps::ClearD(m_compiler, block->bbLiveOut);
-            if (keepAliveThis)
-            {
-                unsigned thisVarIndex = m_compiler->lvaGetDesc(m_compiler->info.compThisArg)->lvVarIndex;
-                VarSetOps::AddElemD(m_compiler, block->bbLiveOut, thisVarIndex);
-            }
-
-            if (block->HasPotentialEHSuccs(m_compiler))
-            {
-                block->VisitEHSuccs(m_compiler, [=](BasicBlock* succ) {
-                    VarSetOps::UnionD(m_compiler, block->bbLiveOut, succ->bbLiveIn);
-                    return BasicBlockVisit::Continue;
-                });
-            }
-
-            VarSetOps::Assign(m_compiler, block->bbLiveIn, block->bbLiveOut);
+            m_compiler->fgSetThrowHelpBlockLiveness(block);
         }
     }
 
@@ -1615,6 +1600,34 @@ void Compiler::fgAddHandlerLiveVars(BasicBlock* block, VARSET_TP& ehHandlerLiveV
     });
 }
 
+//------------------------------------------------------------------------
+// fgSetThrowHelpBlockLiveness: set liveness for throw helper block
+//
+// Arguments:
+//  block -- potential throw helper block
+//
+void Compiler::fgSetThrowHelpBlockLiveness(BasicBlock* block)
+{
+    VarSetOps::ClearD(this, block->bbLiveOut);
+
+    const bool keepAliveThis = lvaKeepAliveAndReportThis() && lvaTable[info.compThisArg].lvTracked;
+    if (keepAliveThis)
+    {
+        unsigned thisVarIndex = lvaGetDesc(info.compThisArg)->lvVarIndex;
+        VarSetOps::AddElemD(this, block->bbLiveOut, thisVarIndex);
+    }
+
+    if (block->HasPotentialEHSuccs(this))
+    {
+        block->VisitEHSuccs(this, [=](BasicBlock* succ) {
+            VarSetOps::UnionD(this, block->bbLiveOut, succ->bbLiveIn);
+            return BasicBlockVisit::Continue;
+        });
+    }
+
+    VarSetOps::Assign(this, block->bbLiveIn, block->bbLiveOut);
+}
+
 #ifdef DEBUG
 
 void Compiler::fgDispBBLiveness()