Unused liveins are not killed

[martin-kjellin]

Consider this MIR file (for Hexagon):

--- |
; ModuleID = (...)
...

name: minimal
body: |
bb.0:
liveins: %r2, %r4
exit
%0 = COPY %r4
%1 = COPY %r2

...

I would expect the liveins to get killed, since they are not really used in the function (and thus not really live), but they are not. Linting gives the following error messages:
allTemporariesUsed: t0:r2 is not used
allTemporariesUsed: t1:r4 is not used

If the COPY instructions are left out, things work as expected.