Fix PR26655: Bail out if all regs of an inst BUNDLE have the correct kill flag

Summary:
While setting kill flags on instructions inside a BUNDLE, we bail out as soon
as we set kill flag on a register.  But we are missing a check when all the
registers already have the correct kill flag set. We need to bail out in that
case as well.

This patch refactors the old code and simply makes use of the addRegisterKilled
function in MachineInstr.cpp in order to determine whether to set/remove kill
on an instruction.

Reviewers: apazos, t.p.northover, pete, MatzeB

Subscribers: MatzeB, davide, llvm-commits

Differential Revision: http://reviews.llvm.org/D17356

llvm-svn: 269092