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/Generator.cpp

@@ -798,4 +798,14 @@ void Generator::set_local_initialized(u32 local_index)
     m_initialized_locals.set(local_index);
 }
 
+Operand Generator::get_this(Optional<Operand> preferred_dst)
+{
+    if (m_current_basic_block->this_().has_value())
+        return m_current_basic_block->this_().value();
+    auto dst = preferred_dst.has_value() ? preferred_dst.value() : Operand(allocate_register());
+    emit<Bytecode::Op::ResolveThisBinding>(dst);
+    m_current_basic_block->set_this(dst);
+    return dst;
+}
+
 }