MASKL(0)/MASKR(0) return -1 instead of 0

Repro:
```f90
  integer :: n
  n = 0
  ! Expects 0, gets -1
  print *, maskl(n)
  print *, maskr(n)
end
```

MASK intrinsic lowering was just implemented in https://github.com/llvm/llvm-project/commit/71ee3570f46cbbbaf08f98d4338ff0946e7dd2b8. The results in the zero case appears to be wrong.