glsl: Avoid calling get_array_element for scalar constants

Accessing scalar constant as an array in function call or
initializer list triggered assert in get_array_element.
Examples:
   func(0[0]);
   vec2 t = { 0[0], 0 };

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107550



Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
(cherry picked from commit 479a849a)