Minor refactor of TcExpr.tcApp
Minor refactor of TcExpr.tcApp

This refactoring has no change in behaviour but makes the
structure clearer

shlevy/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc