Vector imm setting instructions like
V_SET0B/V_SET0H/V_SET0 V_SETALLONESB/V_SETALLONESH/V_SETALLONES XXLXORz/XXLXORspz/XXLXORdpz
Should behave like LI8.
We should set corresponding flags to allow rematerialization and other opts in LICM, RA, Scheduling etc.