Just like we do with undef flags, we need to drop kill flags if they aren't
present in all merged instructions. Otherwise we would merge
<instr1> $r0 <instr2> killed $r0
<instr1> killed $r0 <instr2> undef $r0
<instr1> killed $r0 <instr2> $r0
and then the verifier would complain about $r0 not being live at <instr2>.