An SoImm 32 bit immediate constant is an 8 bit value 0 to 255
rotated right by a 4-bit even count, 0, 2, . . 28, 30.
Most negative values that are multiple of 4 in the range -1020 to -4 cannot be represent by an SoImm.
This commit stops considering an SoImm encoding for a negative offset to a constant island.