glsl: refactor get_variable_being_redeclared() to return always an ir_variable pointer

glsl: refactor get_variable_being_redeclared() to return always an ir_variable pointer

It will return the current variable ('var') or the earlier declaration ('earlier') in
case of redeclaration of that variable.

In order to distinguish between both, 'is_redeclaration' boolean will indicate in which
case we are.

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>