IR node annotation

[naoyam]

Being able to annotate IR nodes with attributes would be handy in IR analyses and transformations.

Motivating example: Tracking expressions initializing reduction buffers

Currently it's done by saving such expressions in a std::unordered_set and passing around it. It's still reasonable but would become problematic if we would need to track many sets of different IR nodes. Alternatively, if we could annotate IR nodes, we could mark initialization expressions as such and use that information later without passing around auxiliary containers. Or, we could just create a new IR node for expressions of a particular trait, however, creating a new different class might be overkill in some cases.

A challenge would be attribute consistency. We'd need to make sure IR attributes are kept valid (or invalidated when necessary). Something we may want to revisit in the future.

[naoyam]

We are sort of doing this by creating temporary maps and sets stored as part of the members of GpuLower.