This is to allow 64 bit constant rematerialization. If a constant
is split into two separate moves initializing sub0 and sub1 like
now RA cannot rematerizalize a 64 bit register.
This gives 10-20% uplift in a set of huge apps heavily using double
precession math.
Fixes: SWDEV-292645
Does this ever return true? If I understand the isel changes, if these conditions were satisfied, you would have selected S_MOV_B64 instead of S_MOV_B64_IMM_PSEUDO.