[MachineCSE] Clear kill-flag on registers imp-def'd by the CSE'd instruction.

Go through implicit defs of CSMI and MI, and clear the kill flags on
their uses in all the instructions between CSMI and MI.
We might have made some of the kill flags redundant, consider:
  subs  ... %NZCV<imp-def>        <- CSMI
  csinc ... %NZCV<imp-use,kill>   <- this kill flag isn't valid anymore
  subs  ... %NZCV<imp-def>        <- MI, to be eliminated
  csinc ... %NZCV<imp-use,kill>
Since we eliminated MI, and reused a register imp-def'd by CSMI
(here %NZCV), that register, if it was killed before MI, should have
that kill flag removed, because it's lifetime was extended.

Also, add an exhaustive testcase for the motivating example.

Reviewed by: Juergen Ributzka <juergen@apple.com>