spirv: Add a helper for getting the NIR type of a vtn_type

spirv: Add a helper for getting the NIR type of a vtn_type

There are a few cases, atomic counters being one example, where the type
used by vtn_ssa_value is not the same as the type we want NIR to use in
derefs and variables.  To solve this, we add a helper which converts
between the types for us.  In the next commit, we'll be adding another
major user of this: images and samplers.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5278>