In addition to basic if-then-else statements, it is possible to write for, while, and do-while loops. In OpenGL ES 2.0, very strict rules governed the usage of loops. Essentially, only loops that could be unrolled by the compiler were supported. These restrictions no longer exist in OpenGL ES 3.0. The GPU hardware is expected to provide support for looping and flow control; thus loops are fully supported.
That is not to say that loops don't come with some performance implications. On most GPU architectures, vertices or fragments are executed in parallel in batches. The GPU typically requires that all fragments or vertices in a batch evaluate all branches (or loop iterations) of flow control statements. If vertices or fragments in a batch execute different paths, then, usually all of the other vertices/fragments in a batch will need to execute that path as well. The size of a batch is GPU dependent and will often require profiling to determine the performance implications of the use of flow control on a particular architecture. However, a good rule of thumb is to try to limit the use of divergent flow control or loop iterations across vertices/fragments