It is covered by lo16-32bit-physreg-copy.mir.
In theory it is reachable on any target with 16 bit instructions (excluding GF11 with True 16 bit instructions). This code converts
Now as to whether the code sequence in that mir test (or any use of a VGPR_LO16 register) is produced in any legitimate shader, I don't think so. That probably needs to be verified empirically on a test corpus, but then this code and I think VGPR_LO16/ VGPR_HI16 can be removed.