integer-gmp: optimise bitBigNat
integer-gmp: optimise bitBigNat

This is a somewhat minor optimisation exploiting the static knowledge
of the operands involved allowing to save a few allocations.

Reviewers: austin, rwbarton, goldfire, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1179

acowley/ghc,elieux/ghc,snoyberg/ghc,vTurbine/ghc,ml9951/ghc,vikraman/ghc,AlexanderPankiv/ghc,ezyang/ghc,ezyang/ghc,vTurbine/ghc,elieux/ghc,wxwxwwxxx/ghc,mettekou/ghc,mettekou/ghc,oldmanmike/ghc,mcschroeder/ghc,GaloisInc/halvm-ghc,acowley/ghc,nkaretnikov/ghc,snoyberg/ghc,ghc-android/ghc,mfine/ghc,sdiehl/ghc,GaloisInc/halvm-ghc,gridaphobe/ghc,acowley/ghc,wxwxwwxxx/ghc,sgillespie/ghc,wxwxwwxxx/ghc,mfine/ghc,olsner/ghc,olsner/ghc,wxwxwwxxx/ghc,wxwxwwxxx/ghc,nushio3/ghc,sgillespie/ghc,AlexanderPankiv/ghc,oldmanmike/ghc,ml9951/ghc,gridaphobe/ghc,oldmanmike/ghc,gridaphobe/ghc,vTurbine/ghc,tjakway/ghcjvm,da-x/ghc,oldmanmike/ghc,ghc-android/ghc,snoyberg/ghc,olsner/ghc,sdiehl/ghc,shlevy/ghc,anton-dessiatov/ghc,da-x/ghc,snoyberg/ghc,nushio3/ghc,nkaretnikov/ghc,ml9951/ghc,nushio3/ghc,olsner/ghc,olsner/ghc,da-x/ghc,mfine/ghc,wxwxwwxxx/ghc,mcschroeder/ghc,da-x/ghc,siddhanathan/ghc,tjakway/ghcjvm,gridaphobe/ghc,tjakway/ghcjvm,shlevy/ghc,mettekou/ghc,ml9951/ghc,GaloisInc/halvm-ghc,shlevy/ghc,da-x/ghc,vikraman/ghc,tjakway/ghcjvm,AlexanderPankiv/ghc,siddhanathan/ghc,nkaretnikov/ghc,ghc-android/ghc,oldmanmike/ghc,nkaretnikov/ghc,olsner/ghc,nkaretnikov/ghc,nushio3/ghc,mcschroeder/ghc,ml9951/ghc,oldmanmike/ghc,gridaphobe/ghc,siddhanathan/ghc,anton-dessiatov/ghc,sgillespie/ghc,shlevy/ghc,anton-dessiatov/ghc,ml9951/ghc,olsner/ghc,GaloisInc/halvm-ghc,mcschroeder/ghc,mettekou/ghc,anton-dessiatov/ghc,GaloisInc/halvm-ghc,sdiehl/ghc,snoyberg/ghc,GaloisInc/halvm-ghc,siddhanathan/ghc,elieux/ghc,mettekou/ghc,anton-dessiatov/ghc,sdiehl/ghc,vikraman/ghc,mcschroeder/ghc,mettekou/ghc,oldmanmike/ghc,elieux/ghc,anton-dessiatov/ghc,shlevy/ghc,nkaretnikov/ghc,nushio3/ghc,nushio3/ghc,sgillespie/ghc,tjakway/ghcjvm,elieux/ghc,sdiehl/ghc,AlexanderPankiv/ghc,AlexanderPankiv/ghc,vTurbine/ghc,vTurbine/ghc,siddhanathan/ghc,sgillespie/ghc,sdiehl/ghc,ezyang/ghc,gridaphobe/ghc,ezyang/ghc,da-x/ghc,acowley/ghc,nushio3/ghc,ezyang/ghc,vikraman/ghc,acowley/ghc,shlevy/ghc,wxwxwwxxx/ghc,ezyang/ghc,siddhanathan/ghc,mfine/ghc,acowley/ghc,snoyberg/ghc,ghc-android/ghc,mcschroeder/ghc,da-x/ghc,ghc-android/ghc,vikraman/ghc,ml9951/ghc,sgillespie/ghc,snoyberg/ghc,anton-dessiatov/ghc,mfine/ghc,mfine/ghc,nkaretnikov/ghc,tjakway/ghcjvm,ml9951/ghc,vTurbine/ghc,ghc-android/ghc,vTurbine/ghc,siddhanathan/ghc,acowley/ghc,tjakway/ghcjvm,ghc-android/ghc,mfine/ghc,sgillespie/ghc,elieux/ghc,mcschroeder/ghc,mettekou/ghc,ezyang/ghc,gridaphobe/ghc,AlexanderPankiv/ghc,shlevy/ghc,vikraman/ghc,vikraman/ghc,GaloisInc/halvm-ghc,AlexanderPankiv/ghc,elieux/ghc,sdiehl/ghc