Optimize add/set/delete operations for string keys in Maps and Sets

This was previously landed in commit 66e2f60, but failed the
collections mjsunit test with --deopt-every-n=1 due to a typo
in the shrinking code. This patch corrects and simplifies the
shrinking logic, and the tests now pass.

R=dslomov@chromium.org

Review URL: https://codereview.chromium.org/777663003

Cr-Commit-Position: refs/heads/master@{#25695}