Preserve "this" for grouped optional calls.

Preserve "this" for grouped optional calls.

Previously, grouped optional calls like (o?.m)() resolved the callee
through the optional chain but dispatched via plain FUNCTION_CALL,
losing the original receiver.

The fix stores the receiver on the call node so the upcoming
call-argument reorder can emit METHOD_FRAME with an explicit "this".
Call-expression setup and optional-chain preserve lookup are routed
through named helpers with generator-side validation.