The GCC 4.0.0 documentation, page 71 in the PostScript version, 
<http://gcc.gnu.org/onlinedocs/gcc-4.0.0/gcc/Optimize-Options.html#Optimize-Options>
in the HTML version, says the following:

-ftree-dce
Perform dead code elimination (DCE) on trees. This flag is enabled by default at -O and higher. 
-ftree-dominator-opts
Perform dead code elimination (DCE) on trees. This flag is enabled by default at -O and higher. 

Dominator options seem to be different from dead code, so presumably these are different.  
Alternatively, if they have the same behavior under different names, that should be noted explicitly, as 
it is elsewhere for such options.