Reland of Thread maybe-assigned through the bytecodes.

test/cctest/interpreter/bytecode_expectations/LetVariableContextSlot.golden

 #
 # Autogenerated by generate-bytecode-expectations.
 #
 
 ---
 wrap: yes
 
 ---
 snippet: "
   let x = 10; function f1() {return x;}
@@ skipping 30 matching lines @@
 bytecodes: [
                 B(CreateFunctionContext), U8(1),
                 B(PushContext), R(1),
                 B(LdaTheHole),
                 B(StaCurrentContextSlot), U8(4),
                 B(CreateClosure), U8(0), U8(2), U8(2),
                 B(Star), R(0),
   /*   30 E> */ B(StackCheck),
   /*   42 S> */ B(LdaSmi), I8(10),
   /*   42 E> */ B(StaCurrentContextSlot), U8(4),
-  /*   72 S> */ B(LdaCurrentContextSlot), U8(4),
+  /*   72 S> */ B(LdaImmutableCurrentContextSlot), U8(4),
   /*   82 S> */ B(Return),
 ]
 constant pool: [
   SHARED_FUNCTION_INFO_TYPE,
 ]
 handlers: [
 ]
 
 ---
 snippet: "
@@ skipping 51 matching lines @@
   /*   48 E> */ B(StaCurrentContextSlot), U8(4),
                 B(LdaUndefined),
   /*   80 S> */ B(Return),
 ]
 constant pool: [
   SHARED_FUNCTION_INFO_TYPE,
 ]
 handlers: [
 ]