glsl: mark variable as loop constant when it is set read only

glsl: mark variable as loop constant when it is set read only

Patch modifies is_loop_constant() to take advantage of 'read_only' bit
in ir_variable to detect a loop constant. Variables marked read-only
are loop constant like mentioned by a comment in the function.

v2: remove unnecessary comment (Francisco)

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=82537
Tested-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>