Revert "Build the substitution correctly in piResultTy"
Revert "Build the substitution correctly in piResultTy"

This reverts commit dbf72dbc6e49b3db7f2337a7a41e95c1d0169163.
This commit introduced performance regressions:
https://ghc.haskell.org/trac/ghc/ticket/11371#comment:27,
I will push it again after I fix it.

Test Plan: revert

Reviewers: simonpj, bgamari, simonmar, austin, goldfire, thomie

Differential Revision: https://phabricator.haskell.org/D1907

oldmanmike/ghc,snoyberg/ghc,vTurbine/ghc,snoyberg/ghc,tjakway/ghcjvm,shlevy/ghc,oldmanmike/ghc,sgillespie/ghc,mcschroeder/ghc,oldmanmike/ghc,sgillespie/ghc,ezyang/ghc,vikraman/ghc,snoyberg/ghc,ezyang/ghc,sdiehl/ghc,snoyberg/ghc,mettekou/ghc,vTurbine/ghc,vTurbine/ghc,mcschroeder/ghc,olsner/ghc,ezyang/ghc,tjakway/ghcjvm,mettekou/ghc,olsner/ghc,tjakway/ghcjvm,shlevy/ghc,snoyberg/ghc,mettekou/ghc,mettekou/ghc,ezyang/ghc,vikraman/ghc,mettekou/ghc,mcschroeder/ghc,vTurbine/ghc,sdiehl/ghc,mcschroeder/ghc,vTurbine/ghc,sgillespie/ghc,vTurbine/ghc,sdiehl/ghc,sgillespie/ghc,vikraman/ghc,oldmanmike/ghc,sgillespie/ghc,vikraman/ghc,tjakway/ghcjvm,sdiehl/ghc,vikraman/ghc,ezyang/ghc,olsner/ghc,shlevy/ghc,snoyberg/ghc,sdiehl/ghc,oldmanmike/ghc,sdiehl/ghc,oldmanmike/ghc,oldmanmike/ghc,mcschroeder/ghc,tjakway/ghcjvm,shlevy/ghc,sgillespie/ghc,mcschroeder/ghc,mcschroeder/ghc,olsner/ghc,sdiehl/ghc,mettekou/ghc,shlevy/ghc,shlevy/ghc,vikraman/ghc,snoyberg/ghc,vTurbine/ghc,ezyang/ghc,olsner/ghc,olsner/ghc,shlevy/ghc,olsner/ghc,tjakway/ghcjvm,tjakway/ghcjvm,vikraman/ghc,ezyang/ghc,sgillespie/ghc,mettekou/ghc