JIT: Create side effect commas as TYP_VOID in block morph (#85320)

This code could create IR shapes like

COMMA struct
  ...
  ASG struct
    dst
    src

After liveness that would sometimes result in

COMMA struct
  ...
  NOP void

which could confuse optimization phases when JitOptRepeat was enabled.

The more canonical shape for these side-effecting GT_COMMA nodes has
them TYP_VOID typed as shown by gtExtractSideEffList, so do the same in
block morph.

Fix #85037

Author: jakobbotsch

src/coreclr/jit/morphblock.cpp

@@ -160,7 +160,7 @@ GenTree* MorphInitBlockHelper::Morph()
             commaPool      = commaPool->gtNext;
 
             assert(comma->OperIs(GT_COMMA));
-            comma->gtType        = m_result->TypeGet();
+            comma->gtType        = TYP_VOID;
             comma->AsOp()->gtOp1 = sideEffects;
             comma->AsOp()->gtOp2 = m_result;
             comma->gtFlags       = (sideEffects->gtFlags | m_result->gtFlags) & GTF_ALL_EFFECT;
@@ -169,7 +169,7 @@ GenTree* MorphInitBlockHelper::Morph()
         }
         else
         {
-            m_result = m_comp->gtNewOperNode(GT_COMMA, m_result->TypeGet(), sideEffects, m_result);
+            m_result = m_comp->gtNewOperNode(GT_COMMA, TYP_VOID, sideEffects, m_result);
         }
         INDEBUG(m_result->gtDebugFlags |= GTF_DEBUG_NODE_MORPHED);