[MIR-Canon] Fixing non-determinism that was breaking bots (NFC).

An earlier fix of a subtle iterator invalidation bug had uncovered a
nondeterminism that was present in the MultiUsers bag. Problem was that
MultiUsers was being looked up using pointers.

This patch is an NFC change that numbers each multiuser and processes each in
numbered order. This fixes the test failure on netbsd and will likely fix the
green-dragon bot too.

llvm-svn: 363012