Can C++ compilers optimize "if" statements inside "for" loops?

Consider an example like this:

if (flag)
  for (condition)
    do_something();
else
  for (condition)
    do_something_else();

If flag doesn't change inside the for loops, this should be semantically equivalent to:

for (condition)
  if (flag)
    do_something();
  else
    do_something_else();

Only in the first case, the code might be much longer (e.g. if several for loops are used or if do_something() is a code block that is mostly identical to do_something_else()), while in the second case, the flag gets checked many times.

I'm curious whether current C++ compilers (most importantly, g++) would be able to optimize the second example to get rid of the repeated tests inside the for loop. If so, under what conditions is this possible?