refactor(library/compiler/cse): isolate cse processor procedure
refactor(library/compiler/cse): isolate cse processor procedure

We will use it to fix a performance problem in case expressions for
inductive datatypes that have constructors without data.

digama0/lean,Kha/lean,leanprover/lean,Kha/lean,leodemoura/lean,leanprover-community/lean,leanprover-community/lean,leanprover/lean,fpvandoorn/lean,fpvandoorn/lean,leanprover-community/lean,leanprover-community/lean,dselsam/lean,leodemoura/lean,digama0/lean,dselsam/lean,leanprover/lean,leodemoura/lean,digama0/lean,dselsam/lean,leanprover/lean,leodemoura/lean,fpvandoorn/lean,Kha/lean,Kha/lean,digama0/lean,leodemoura/lean,Kha/lean,digama0/lean,leanprover-community/lean,fpvandoorn/lean,leodemoura/lean,leanprover-community/lean,dselsam/lean,Kha/lean,fpvandoorn/lean,dselsam/lean,dselsam/lean,fpvandoorn/lean,leanprover/lean,leanprover/lean,digama0/lean