improve user split for x-shard users
improve user split for x-shard users

when users have matching local ids across shard, we were trying to make
some guesses that aren't valid.

refs FOO-2307
flag = none

test plan:
- merge two users with the same local id x-shard
- try to split them
- it should work

Change-Id: I62e62368353be5d51a92665e40ede05589b60b9d
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/272255
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
QA-Review: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
Product-Review: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
Reviewed-by: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms