Review checklist: suggest checking insertions expected to be new. (#7245)

Suggest checking that PRs assert that insertions into sets or maps
expected to be adding new values didn't actually just replace some
existing value.

Bug #7048 and its several duplicates would have been caught sooner if
the insertion of the new spill temporary into the `spilled_composites`
table had asserted that there was no existing spill variable for that
expression.

Author: jimblandy

docs/review-checklist.md

@@ -34,6 +34,9 @@ satisfying way to address in a more robust way.
 - [ ] If your change iterates over a collection, did you ensure the
       order of iteration was deterministic? Using `HashMap` and
       `HashSet` is fine, as long as you don't iterate over it.
+- [ ] If you insert elements into a set or map that you expect are not
+      already present, did you make an assertion about `insert`'s
+      return value?
 
 ### WGSL Extensions