JIT: postorder local assertion prop for fieldwise block op indirs (#118368)

After we form the block op field indirs, run postorder local assertion
prop on them, so that they see the same amount of assertion prop as
other indirs.

Fixes (parts of) #115789.

Author: AndyAyersMS

src/coreclr/jit/morphblock.cpp

@@ -1164,6 +1164,32 @@ GenTree* MorphCopyBlockHelper::CopyFieldByField()
         addrSpillStore->SetMorphed(m_comp);
     }
 
+    auto postOrderAssertionProp = [=](GenTree* tree) {
+        if (m_comp->fgGlobalMorph && m_comp->optLocalAssertionProp && (m_comp->optAssertionCount > 0))
+        {
+            GenTree* optimizedTree = tree;
+            bool     again         = JitConfig.JitEnablePostorderLocalAssertionProp() > 0;
+            bool     didOptimize   = false;
+
+            while (again)
+            {
+                tree          = optimizedTree;
+                optimizedTree = m_comp->optAssertionProp(m_comp->apLocalPostorder, tree, nullptr, nullptr);
+                again         = (optimizedTree != nullptr);
+                didOptimize |= again;
+            }
+
+            assert(tree != nullptr);
+
+            if (didOptimize)
+            {
+                m_comp->gtUpdateNodeSideEffects(tree);
+            }
+        }
+
+        return tree;
+    };
+
     auto grabAddr = [=, &result](unsigned offs) {
         GenTree* addrClone = nullptr;
         // Need address of the source.
@@ -1316,6 +1342,8 @@ GenTree* MorphCopyBlockHelper::CopyFieldByField()
             }
         }
         assert(srcFld != nullptr);
+
+        srcFld = postOrderAssertionProp(srcFld);
         srcFld->SetMorphed(m_comp);
 
         GenTree* dstFldStore;
@@ -1372,7 +1400,16 @@ GenTree* MorphCopyBlockHelper::CopyFieldByField()
                 }
             }
         }
-        noway_assert(dstFldStore->TypeGet() == srcFld->TypeGet());
+
+        // Allow mismatched types only when srcFld is an integer constant
+        //
+        if (dstFldStore->TypeGet() != srcFld->TypeGet())
+        {
+            noway_assert(genActualType(dstFldStore->TypeGet()) == genActualType(srcFld->TypeGet()));
+            assert(srcFld->IsIntegralConst());
+        }
+
+        dstFldStore = postOrderAssertionProp(dstFldStore);
         dstFldStore->SetMorphed(m_comp);
 
         if (m_comp->optLocalAssertionProp)