glsl: Don't attempt to do dead varying elimination on gl_LastFragData arrays.

glsl: Don't attempt to do dead varying elimination on gl_LastFragData arrays.

Apparently this pass can only handle elimination of a single built-in
fragment output array, so the presence of gl_LastFragData (which it
wouldn't split correctly anyway) could prevent it from splitting the
actual gl_FragData array.  Just match gl_FragData by name since it's
the only built-in it can handle.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>