Splitting off task 3 from #11735. When compiling https://ghc.haskell.org/trac/ghc/attachment/ticket/14683/Grammar.hs, simplCast eats up more execution time than we think it should.

From https://ghc.haskell.org/trac/ghc/ticket/11735#comment:10:

Something is clearly wrong with Simplify.simplCast. I think I know what it is. Given

(fun |> co) @t1 @t2 ... @tn

we will call pushCoTyArg n times, and hence does n singleton substitutions, via the n calls to piResultTy.

Solution: gather up those type arguments (easy) and define

pushCoTyArgs :: Coercion -> [Type] -> Maybe ([Type], Coercion)

And https://ghc.haskell.org/trac/ghc/ticket/11735#comment:41:

OK. I looked at pushCoTyArg and friends, and I have a very simple solution: just move the isReflexiveCo case in addCoerce (a local function within Simplify.simplCast) to the top. That should do it. Then pushCoTyArg is never called with a reflexive coercion, and so the piResultTy case won't happen.

Now, pushCoArgs might still call pushCoTyArg with a reflexive coercion, but it can be taught not to as well: Have pushCoArgs return a Maybe ([CoreArg], Maybe Coercion) and pushCoArg return a Maybe (CoreArg, Maybe Coercion). If the second return values are Nothing, that means that there is no cast (i.e., that the cast would have been reflexive). The only client of pushCoArg(s) is exprIsConApp_maybe, which simply omits a cast if pushCoArgs returns Nothing. Then, we never have to bother creating the reflexive coercions.

This should be an easy win all around.