Modified _eval_expand_multinomial.
Modified _eval_expand_multinomial.

When expanding instances of Pow with a negative integer power, we don't need to
call .expand(), instead we just need to call _eval_expand_multinomial().  This
patch reflects this change and should speed things up a bit.

Signed-off-by: Ondrej Certik <b816faa87b7d35274d2e545c5be11ed4376f3ccf@certik.cz>

lidavidm/sympy,liangjiaxing/sympy,shipci/sympy,toolforger/sympy,lidavidm/sympy,jbbskinny/sympy,pernici/sympy,dqnykamp/sympy,Mitchkoens/sympy,garvitr/sympy,ga7g08/sympy,wyom/sympy,skidzo/sympy,Titan-C/sympy,yashsharan/sympy,jbbskinny/sympy,MechCoder/sympy,kaichogami/sympy,souravsingh/sympy,lindsayad/sympy,farhaanbukhsh/sympy,cswiercz/sympy,MridulS/sympy,skidzo/sympy,beni55/sympy,saurabhjn76/sympy,AkademieOlympia/sympy,debugger22/sympy,kaushik94/sympy,pandeyadarsh/sympy,sahmed95/sympy,Arafatk/sympy,cccfran/sympy,srjoglekar246/sympy,shipci/sympy,ga7g08/sympy,Gadal/sympy,sampadsaha5/sympy,grevutiu-gabriel/sympy,moble/sympy,abhiii5459/sympy,liangjiaxing/sympy,lidavidm/sympy,atreyv/sympy,Shaswat27/sympy,Curious72/sympy,abloomston/sympy,Vishluck/sympy,kmacinnis/sympy,wanglongqi/sympy,iamutkarshtiwari/sympy,Gadal/sympy,kevalds51/sympy,shikil/sympy,AunShiLord/sympy,abloomston/sympy,atsao72/sympy,sunny94/temp,sunny94/temp,debugger22/sympy,kumarkrishna/sympy,Curious72/sympy,hargup/sympy,jbaayen/sympy,Davidjohnwilson/sympy,chaffra/sympy,aktech/sympy,jaimahajan1997/sympy,kaushik94/sympy,sunny94/temp,meghana1995/sympy,madan96/sympy,Designist/sympy,Davidjohnwilson/sympy,KevinGoodsell/sympy,kaichogami/sympy,drufat/sympy,bukzor/sympy,emon10005/sympy,lindsayad/sympy,sampadsaha5/sympy,wyom/sympy,yukoba/sympy,ahhda/sympy,Mitchkoens/sympy,souravsingh/sympy,Arafatk/sympy,toolforger/sympy,garvitr/sympy,vipulroxx/sympy,atreyv/sympy,shikil/sympy,grevutiu-gabriel/sympy,jbbskinny/sympy,diofant/diofant,maniteja123/sympy,Davidjohnwilson/sympy,postvakje/sympy,pandeyadarsh/sympy,rahuldan/sympy,abloomston/sympy,Arafatk/sympy,mafiya69/sympy,shikil/sympy,cswiercz/sympy,flacjacket/sympy,amitjamadagni/sympy,madan96/sympy,jerli/sympy,pbrady/sympy,Vishluck/sympy,Designist/sympy,Shaswat27/sympy,sahilshekhawat/sympy,jaimahajan1997/sympy,hrashk/sympy,moble/sympy,oliverlee/sympy,amitjamadagni/sympy,ahhda/sympy,Sumith1896/sympy,ChristinaZografou/sympy,jamesblunt/sympy,meghana1995/sympy,minrk/sympy,Titan-C/sympy,cswiercz/sympy,MridulS/sympy,aktech/sympy,mattpap/sympy-polys,AunShiLord/sympy,debugger22/sympy,mafiya69/sympy,emon10005/sympy,ahhda/sympy,minrk/sympy,MechCoder/sympy,iamutkarshtiwari/sympy,shipci/sympy,mcdaniel67/sympy,hrashk/sympy,farhaanbukhsh/sympy,sahilshekhawat/sympy,abhiii5459/sympy,ga7g08/sympy,saurabhjn76/sympy,Shaswat27/sympy,AunShiLord/sympy,dqnykamp/sympy,kumarkrishna/sympy,toolforger/sympy,saurabhjn76/sympy,hargup/sympy,mcdaniel67/sympy,Vishluck/sympy,postvakje/sympy,cccfran/sympy,ChristinaZografou/sympy,Sumith1896/sympy,ChristinaZografou/sympy,VaibhavAgarwalVA/sympy,rahuldan/sympy,pbrady/sympy,yashsharan/sympy,kumarkrishna/sympy,rahuldan/sympy,VaibhavAgarwalVA/sympy,asm666/sympy,AkademieOlympia/sympy,jamesblunt/sympy,bukzor/sympy,jamesblunt/sympy,mcdaniel67/sympy,sahilshekhawat/sympy,sahmed95/sympy,asm666/sympy,atreyv/sympy,mafiya69/sympy,sampadsaha5/sympy,kevalds51/sympy,vipulroxx/sympy,dqnykamp/sympy,bukzor/sympy,maniteja123/sympy,skirpichev/omg,kmacinnis/sympy,Sumith1896/sympy,Gadal/sympy,Curious72/sympy,grevutiu-gabriel/sympy,beni55/sympy,atsao72/sympy,maniteja123/sympy,drufat/sympy,wanglongqi/sympy,garvitr/sympy,wanglongqi/sympy,hrashk/sympy,kaushik94/sympy,meghana1995/sympy,skidzo/sympy,MechCoder/sympy,moble/sympy,beni55/sympy,postvakje/sympy,MridulS/sympy,abhiii5459/sympy,oliverlee/sympy,tovrstra/sympy,yashsharan/sympy,lindsayad/sympy,farhaanbukhsh/sympy,pbrady/sympy,kmacinnis/sympy,sahmed95/sympy,oliverlee/sympy,kaichogami/sympy,jaimahajan1997/sympy,Titan-C/sympy,iamutkarshtiwari/sympy,chaffra/sympy,pandeyadarsh/sympy,jerli/sympy,souravsingh/sympy,wyom/sympy,chaffra/sympy,jerli/sympy,drufat/sympy,Mitchkoens/sympy,hargup/sympy,kevalds51/sympy,AkademieOlympia/sympy,aktech/sympy,fperez/sympy,VaibhavAgarwalVA/sympy,madan96/sympy,vipulroxx/sympy,Designist/sympy,yukoba/sympy,asm666/sympy,cccfran/sympy,atsao72/sympy,yukoba/sympy,emon10005/sympy,liangjiaxing/sympy