When var_type_feedback is nullptr and {lhs} is not a String,
rhs_instance_type is not needed.

Bug: 
Change-Id: Ia39aa39ec7ad0063d2fb9b01fd326d7993ab4d26
Reviewed-on: https://chromium-review.googlesource.com/659340
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48086}