If you wanted the values only and not the IF formulas then

Sub NewLoop()
    Dim Rg         As Range
     'there may be no blank cells
    On Error Resume Next
     'Find all cells in column B that have a corresponding blank cell in column E in the usedrange
    Set Rg = Intersect(Range("E:E"), ActiveSheet.UsedRange, Cells.SpecialCells(xlCellTypeBlanks)).Offset(0, -3)
    On Error GoTo 0
     'Add the formula to the B cells and the XX to E cells
    If Not Rg Is Nothing Then
        Rg.FormulaR1C1 = "=IF(RC[-1]=R[-1]C[-1],R[-1]C+RC[1],RC[1])"
        Rg.Formula = Rg.Value
        Rg.Offset(0, 3) = "XX"
    End If
End Sub
Cheers

Dave