GLSL: Minor nit, check flushed_phi_variables with count().
GLSL: Minor nit, check flushed_phi_variables with count().

KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross