[IRSim] Ensure that assignment accurately reduces potential mapping between different candidates

Previous:
When we do not make decisions about commutative operands, we can end up in a situation where two values have two potential canonical numbers between two regions. This ensures that an ordering is decided after the initial structure between two regions is determined.

Current:
Previously the outliner only checked that assignment to a value matched what was already known, this patch makes sure that it matches what has already been found, and creates a mapping between the two values where it is a one-to-one mapping.

Reviewer: paquette
Differential Revision: https://reviews.llvm.org/D139336