Reland of Thread maybe-assigned through the bytecodes.

src/interpreter/bytecode-array-builder.cc

Index: src/interpreter/bytecode-array-builder.cc
diff --git a/src/interpreter/bytecode-array-builder.cc b/src/interpreter/bytecode-array-builder.cc
index 6f506446ec6bd4fbe9f4f7280098bc22d7bdb7f6..c0ff19ac77f2c413ed245ef796db19eb6811fe75 100644
--- a/src/interpreter/bytecode-array-builder.cc
+++ b/src/interpreter/bytecode-array-builder.cc
@@ -477,12 +477,20 @@
   return *this;
 }
 
-BytecodeArrayBuilder& BytecodeArrayBuilder::LoadContextSlot(Register context,
-                                                            int slot_index,
-                                                            int depth) {
+BytecodeArrayBuilder& BytecodeArrayBuilder::LoadContextSlot(
+    Register context, int slot_index, int depth,
+    ContextSlotMutability mutability) {
   if (context.is_current_context() && depth == 0) {
-    OutputLdaCurrentContextSlot(slot_index);
-  } else {
+    if (mutability == kImmutableSlot) {
+      OutputLdaImmutableCurrentContextSlot(slot_index);
+    } else {
+      DCHECK_EQ(kMutableSlot, mutability);
+      OutputLdaCurrentContextSlot(slot_index);
+    }
+  } else if (mutability == kImmutableSlot) {
+    OutputLdaImmutableContextSlot(context, slot_index, depth);
+  } else {
+    DCHECK_EQ(mutability, kMutableSlot);
     OutputLdaContextSlot(context, slot_index, depth);
   }
   return *this;