nv50: Clear nv50_ir_prog_info of dead and codegen specific variables

These variables are either not used in the code, only assigned but never accessed, or only used inside codegen. Another reason is that this patch will be preceding shader cache, and these variables are useless to cache. Removing/moving them should make it clearer by removing the case something from the structure is not cached.

Shader cache patch: !4264 (merged)