[IRSim] Ignore debug instructions when creating canonical numbering

When constructing canonical relationships between two regions, the first instruction of a basic block from the first region is used to find the corresponding basic block from the second region. However, debug instructions are not included in similarity matching, and therefore do not have a canonical numbering. This patch makes sure to ignore the debug instructions when finding the first instruction in a basic block.

Reviewer: paquette

Differential Revision: https://reviews.llvm.org/D123903