Fix bug in readMVar implementation: keep clean MVars clean.
Fix bug in readMVar implementation: keep clean MVars clean.

The readMVar implementation had only partially implemented
a micro-optimization which allows us to avoid adding an MVar
to the mutable list if the MVar was not changed.  However,
this was not applied to the release method on the fast path,
resulting in dirty MVars which were not added to the mutable list.

Signed-off-by: Edward Z. Yang <dbd597f5635f432486c5d365e9bb585b3eaa1853@mit.edu>

christiaanb/ghc,hferreiro/replay,tibbe/ghc,tibbe/ghc,sgillespie/ghc,tibbe/ghc,gcampax/ghc,oldmanmike/ghc,elieux/ghc,gcampax/ghc,hferreiro/replay,olsner/ghc,ghc-android/ghc,nushio3/ghc,jstolarek/ghc,spacekitteh/smcghc,tjakway/ghcjvm,nkaretnikov/ghc,siddhanathan/ghc,nathyong/microghc-ghc,mettekou/ghc,jstolarek/ghc,ml9951/ghc,wxwxwwxxx/ghc,AlexanderPankiv/ghc,shlevy/ghc,shlevy/ghc,ghc-android/ghc,mcschroeder/ghc,snoyberg/ghc,fmthoma/ghc,holzensp/ghc,olsner/ghc,spacekitteh/smcghc,christiaanb/ghc,sgillespie/ghc,da-x/ghc,gridaphobe/ghc,GaloisInc/halvm-ghc,christiaanb/ghc,siddhanathan/ghc,oldmanmike/ghc,holzensp/ghc,tjakway/ghcjvm,vikraman/ghc,shlevy/ghc,lukexi/ghc-7.8-arm64,nushio3/ghc,nkaretnikov/ghc,fmthoma/ghc,mcschroeder/ghc,siddhanathan/ghc,snoyberg/ghc,sgillespie/ghc,olsner/ghc,da-x/ghc,forked-upstream-packages-for-ghcjs/ghc,mfine/ghc,gcampax/ghc,vTurbine/ghc,anton-dessiatov/ghc,gridaphobe/ghc,jstolarek/ghc,gridaphobe/ghc,GaloisInc/halvm-ghc,green-haskell/ghc,mcschroeder/ghc,ml9951/ghc,lukexi/ghc,TomMD/ghc,shlevy/ghc,da-x/ghc,tjakway/ghcjvm,ml9951/ghc,spacekitteh/smcghc,frantisekfarka/ghc-dsi,ezyang/ghc,snoyberg/ghc,nkaretnikov/ghc,holzensp/ghc,ezyang/ghc,jstolarek/ghc,lukexi/ghc-7.8-arm64,gridaphobe/ghc,vTurbine/ghc,lukexi/ghc,ekmett/ghc,jstolarek/ghc,ryantm/ghc,vTurbine/ghc,siddhanathan/ghc,sgillespie/ghc,vTurbine/ghc,bitemyapp/ghc,lukexi/ghc-7.8-arm64,elieux/ghc,acowley/ghc,vTurbine/ghc,sgillespie/ghc,da-x/ghc,olsner/ghc,gridaphobe/ghc,mcschroeder/ghc,acowley/ghc,snoyberg/ghc,anton-dessiatov/ghc,vikraman/ghc,GaloisInc/halvm-ghc,mfine/ghc,wxwxwwxxx/ghc,siddhanathan/ghc,ml9951/ghc,shlevy/ghc,fmthoma/ghc,forked-upstream-packages-for-ghcjs/ghc,sdiehl/ghc,wxwxwwxxx/ghc,spacekitteh/smcghc,bitemyapp/ghc,ghc-android/ghc,forked-upstream-packages-for-ghcjs/ghc,christiaanb/ghc,nkaretnikov/ghc,mettekou/ghc,GaloisInc/halvm-ghc,mettekou/ghc,tjakway/ghcjvm,mcschroeder/ghc,fmthoma/ghc,TomMD/ghc,acowley/ghc,tibbe/ghc,AlexanderPankiv/ghc,ekmett/ghc,mfine/ghc,nathyong/microghc-ghc,nushio3/ghc,nkaretnikov/ghc,bitemyapp/ghc,ml9951/ghc,vikraman/ghc,elieux/ghc,nathyong/microghc-ghc,snoyberg/ghc,vikraman/ghc,sgillespie/ghc,forked-upstream-packages-for-ghcjs/ghc,green-haskell/ghc,AlexanderPankiv/ghc,mfine/ghc,fmthoma/ghc,ghc-android/ghc,acowley/ghc,da-x/ghc,christiaanb/ghc,GaloisInc/halvm-ghc,gcampax/ghc,TomMD/ghc,lukexi/ghc,christiaanb/ghc,ml9951/ghc,ryantm/ghc,acowley/ghc,urbanslug/ghc,vTurbine/ghc,christiaanb/ghc,TomMD/ghc,green-haskell/ghc,wxwxwwxxx/ghc,hferreiro/replay,elieux/ghc,sgillespie/ghc,ezyang/ghc,ryantm/ghc,acowley/ghc,frantisekfarka/ghc-dsi,wxwxwwxxx/ghc,mettekou/ghc,ezyang/ghc,anton-dessiatov/ghc,ezyang/ghc,oldmanmike/ghc,urbanslug/ghc,GaloisInc/halvm-ghc,nushio3/ghc,lukexi/ghc,sdiehl/ghc,urbanslug/ghc,spacekitteh/smcghc,green-haskell/ghc,nathyong/microghc-ghc,forked-upstream-packages-for-ghcjs/ghc,ghc-android/ghc,ezyang/ghc,forked-upstream-packages-for-ghcjs/ghc,nushio3/ghc,ml9951/ghc,ekmett/ghc,ghc-android/ghc,hferreiro/replay,mfine/ghc,gcampax/ghc,da-x/ghc,snoyberg/ghc,nushio3/ghc,nathyong/microghc-ghc,vikraman/ghc,sdiehl/ghc,ml9951/ghc,elieux/ghc,urbanslug/ghc,olsner/ghc,sdiehl/ghc,hferreiro/replay,oldmanmike/ghc,ryantm/ghc,ghc-android/ghc,vikraman/ghc,mcschroeder/ghc,bitemyapp/ghc,shlevy/ghc,urbanslug/ghc,sdiehl/ghc,siddhanathan/ghc,ryantm/ghc,TomMD/ghc,AlexanderPankiv/ghc,sdiehl/ghc,oldmanmike/ghc,frantisekfarka/ghc-dsi,nkaretnikov/ghc,elieux/ghc,nathyong/microghc-ghc,lukexi/ghc,fmthoma/ghc,tjakway/ghcjvm,tjakway/ghcjvm,holzensp/ghc,urbanslug/ghc,lukexi/ghc-7.8-arm64,frantisekfarka/ghc-dsi,nathyong/microghc-ghc,oldmanmike/ghc,bitemyapp/ghc,ekmett/ghc,oldmanmike/ghc,frantisekfarka/ghc-dsi,ezyang/ghc,nkaretnikov/ghc,gcampax/ghc,AlexanderPankiv/ghc,mettekou/ghc,vikraman/ghc,anton-dessiatov/ghc,vTurbine/ghc,nushio3/ghc,anton-dessiatov/ghc,wxwxwwxxx/ghc,green-haskell/ghc,mcschroeder/ghc,gridaphobe/ghc,lukexi/ghc-7.8-arm64,mettekou/ghc,GaloisInc/halvm-ghc,wxwxwwxxx/ghc,snoyberg/ghc,anton-dessiatov/ghc,elieux/ghc,gcampax/ghc,holzensp/ghc,ekmett/ghc,tibbe/ghc,mettekou/ghc,da-x/ghc,urbanslug/ghc,TomMD/ghc,mfine/ghc,sdiehl/ghc,fmthoma/ghc,olsner/ghc,gridaphobe/ghc,shlevy/ghc,hferreiro/replay,olsner/ghc,AlexanderPankiv/ghc,mfine/ghc,tjakway/ghcjvm,AlexanderPankiv/ghc,forked-upstream-packages-for-ghcjs/ghc,anton-dessiatov/ghc,acowley/ghc,TomMD/ghc,siddhanathan/ghc,hferreiro/replay