[gold][patch] Avoid looking up Input_merge_map multiple times

[Cary Coutant]

>> The attached patch (which applies on top of my previous one) avoid
>> looking up Input_merge_map multiple times when we know were are
>> processing items from the same input section.

> A rebased patch is attached.
>
> This is the last refactoring before the gc implementation.

This patch is OK, with a ChangeLog entry.

Thanks!

-cary