Work on more general solution to adding access chain placeholders during reduction

In spirv-reduce's loop-to-conditional reduction pass, if we find that an access chain use is no longer dominated, we replace the use with an access to a (possibly new) variable of the right type. This is fine if the storage class is Private or Function; we can just add a new variable. But for other storage classes I'm unsure whether it's legal to arbitrarily add more global-scoped variables with the given storage class; there might be limits on the number of such variables that are allowed, and quite possibly constraints on adding such variables without modifying the associated host code.

Start in spirv-reduce's loop-to-conditional reduction pass and trace how access chain uses are replaced when they are no longer dominated. Investigate the legality and constraints of adding variables for storage classes other than Private or Function, including possible limits and associated host-code requirements; done means the reduction has a general valid replacement strategy for those cases.

Written by the indexing model from the issue text.