There is a typo

line 1066
if immrot(v) == 0 {
return v, 0
}

it should be
if immrot(v) != 0 {
return v, 0
}

The original intention is
if "v is already imm-12" {
return v, 0
} else {
return y, x // y - x = v
}

But actually become
if "v is not imm-12" {
return v, 0
}

So case 107 of the optab never be triggered.