Keep track of which types of keys need an update and which don't.

Strings need an update because the new key might pin a smaller backing store.
Floats need an update because it might be +0/-0.
Interfaces need an update because they may contain strings or floats.

Fixes #11088

Change-Id: I9ade53c1dfb3c1a2870d68d07201bc8128e9f217
Reviewed-on: https://go-review.googlesource.com/10843Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>