[MachineLateInstrsCleanup] Bugfix for handling of kill flags.

With cb57b7a7, the kill flags are now tracked during the forward search over
the instructions and the call to findRegisterUseOperandIdx() should therefore
only check for killing uses.

As shown with the failing test CodeGen/Hexagon/vector-sint-to-fp.ll, it could
otherwise be the case that an undef use after the instruction that killed the
register will be inserted into MBBKills, and the kill flag will not be
cleared.