[MachineLICM] Clear subregister kill flags (#67240)

When hosting a loop invariant instruction the resulting register must be
live in
all the basic blocks of the loop body and the killed flags of the
register must
be cleared.

Before this patch killed flags of subregister to a hoisted superregister
was not
cleared in the loop body.

This was found in an out of tree target, but the testcase
mlicm-stack-write-check.mir was modified to trigger the case.