We lower known CR bit spills (CRSET/CRUNSET) to load and spill the known value
but forgot to remove the redundant spills.
e.g., This sequence was used to spill a CRUNSET:
crclr 4*cr5+lt mfocrf r3,4 rlwinm r3,r3,20,0,0 stw r3,132(r1)
Custom lowering of known CR bit spills lower it to:
crclr 4*cr5+lt li r3,0 stw r3,132(r1)
crxor is redundant if there is no use of 4*cr5+lt so we should remove it