Threads lockstep and conditions in compute shader
Not all threads will execute in lockstep but they are split into groups whose threads are locked to each other.

This means that if only 1 thread out of all threads enters a branch then only 1 group will need to enter that branch while all the others will skip it.

In that group that has to execute both branches it will actually execute both branches but threads will throw away the result of the branch that it didn't need to go into.

For more information see this question.