glsl: use ir_rvalue instead of ir_dereference in auxiliary functions

glsl: use ir_rvalue instead of ir_dereference in auxiliary functions

In a later commit we will need to handle ir_swizzle nodes too, which are
not an ir_dereference. That can happen, for example, when we pass a
component of an integer vector as argument to any of the SSBO atomic
functions.

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>