OpenCL optimization: removing conditional assignments

Here is a quote from the AMD OpenCL optimization guide:

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Optimizing Kernels for Southern Island GPUs

Remove Conditional Assignments

A conditional of the form “if-then-else” generates branching. Use the select() function to replace these structures with conditional assignments that do not cause branching. For example:

if(x==1) r=0.5;
if(x==2) r=1.0;

becomes

r = select(r, 0.5, x==1);
r = select(r, 1.0, x==2);

Note that if the body of the if statement contains an I/O, the if statement cannot be eliminated.

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I am confused about the last statement:

If the body of the if statement contains an I/O, the if statement cannot be eliminated.

Can someone please explain?