compute_precision() does not understand Literal precisions #3557

I noticed this issue while working on PR #3554.

subroutine sub()
  real(4) :: r
  r = r + 1.0_4
end subroutine

The value of psyir.walk(Assignment)[0].rhs.datatype is Scalar<REAL, UNDEFINED>. This is because the types of the arguments to + have slightly different representations:

• Scalar<REAL, 4>
• Scalar<REAL, Literal[value:'4', Scalar<INTEGER, UNDEFINED>]>

I think the source of the problem is that the compute_precision() function does not understand Literal precisions. It is probably an easy fix.