glsl: ir_explog_to_explog2 is no more

glsl: ir_explog_to_explog2 is no more

Since 63684a9a ("glsl: Combine many instruction lowering passes
into one.", Thu Nov 18 2010), we no longer have anything called
ir_explog_to_explog2. So it's only confusing to have those
references there.

Update with the appropriate method, so people can grep for it in
the current tree if they encounter it.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>