GDP in expenditure approach GDP calculated using expenditure apporach is like this:
GDP=C+I+G+(X-M)
However on this site:
http://www.singstat.gov.sg/statistics/visualising-data/charts/share-of-gdp-by-expenditure

The data is avaialable for C, G and net exports, but there are two variables: Gross Fixed Capital Formation and Changes In Inventories. Since there is no 'I' variable, can I assume that the sum of those two variables are equal I?
The authoritative source here is the United Nations System of National Accounts, SNA.

Change in inventories are indeed part of what we normally call Investment. First, for clarification, the definition of "change in inventories", taken from page 108 of SNA, version 2008:


  The basic principle underlying the measurement of changes
  in  inventories  of  finished  goods  is  that  output  should  be
  recorded at the time it is produced and valued at the same
  price  whether  it  is  sold,  otherwise  used  or  entered  into
  inventories for sale or use later. In effect, goods only enter
  inventories when they are not immediately used
   for sale or other use in the period they
   are produced. Similarly, goods are  withdrawn  from  inventories  when  the  demand  for  the
  goods exceeds the amount produced in a period. No output
  is recorded when goods produced previously are withdrawn
  from  inventories  and  sold  or  otherwise  used  unless  a
  storage activity as described be low in section F takes place. 


Second, the components of "Investment" (officially called Gross capital formation), as stated in page 282 of the aforementioned document is:


  There are three types of capital formation to be examined,
  gross  fixed  capital  formation,
    changes  in  inventories  and
  acquisition less disposal of valuables. 


This and following pages go about explaining each of them, and how to measure them. 

Finally, the document provides a worked example of the table of "uses of output" (the expenditure approach of GDP). Here is a screenshot of a section of that worked example, presented in page 291. You can see that change in inventories are indeed treated as part of "Gross Capital Formation":