Subregister liveness tracking is not implemented for X86 backend, so
sometimes the whole super register is said to be live, when only a
subregister is really live. That might happen if the def and the use
are located in different MBBs, see added fixup-bw-isnt.mir test.
However, using knowledge of the specific instructions handled by the
bw-fixup-pass we can get more precise liveness information which this