[NFC] Introduce range based singleton searches for loop queries.

Summary:
Several loop queries look for a singleton by finding all instances and then
returning whether there is 1 instance or not. This can be improved by
stopping the search after 2 have been found. Introduce generic range
based singleton searches that stop after finding a second value
and use them for these loop queries.

There is no intended functional change other than improved compile-time
efficiency.

Author: Jamie Schmeiser <schmeise@ca.ibm.com>
Reviewed By: Meinersbur (Michael Kruse)
Differential Revision: https://reviews.llvm.org/D136261