i965: wrap unused function in #ifndef NDEBUG
i965: wrap unused function in #ifndef NDEBUG

This function is only ever used by an assert() this fixes an
unused function warning in release builds.

Reviewed-by: Iago Toral Quiroga <a847aabaac44184372f6b1cd1a59aa254daaeb0b@igalia.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler