LibJS/Bytecode: Only emit ResolveThisBinding once per basic block

Once executed, this instruction will always produce the same result
in subsequent executions, so it's okay to cache it.

Unfortunately it may throw, so we can't just hoist it to the top of
every executable, since that would break observable execution order.

Userland/Libraries/LibJS/Bytecode/BasicBlock.h

@@ -9,6 +9,7 @@
 #include <AK/Badge.h>
 #include <AK/String.h>
 #include <LibJS/Bytecode/Executable.h>
+#include <LibJS/Bytecode/Operand.h>
 #include <LibJS/Forward.h>
 #include <LibJS/Heap/Handle.h>
 
@@ -51,6 +52,9 @@ public:
     auto const& source_map() const { return m_source_map; }
     void add_source_map_entry(size_t bytecode_offset, SourceRecord const& source_record) { m_source_map.set(bytecode_offset, source_record); }
 
+    auto const& this_() const { return m_this; }
+    void set_this(Operand operand) { m_this = operand; }
+
 private:
     explicit BasicBlock(u32 index, String name);
 
@@ -62,6 +66,8 @@ private:
     bool m_terminated { false };
 
     HashMap<size_t, SourceRecord> m_source_map;
+
+    Optional<Operand> m_this;
 };
 
 }