glsl: Transform pow(x, 4) into (x*x)*(x*x).
glsl: Transform pow(x, 4) into (x*x)*(x*x).

Reviewed-by: Juha-Pekka Heikkila <cab67866509becb852d2a4e6f7e5924ad04fb975@gmail.com>
Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler