glsl: Add the possibility for ir_variable to have a non-ralloced name

glsl: Add the possibility for ir_variable to have a non-ralloced name

Specifically, ir_var_temporary variables constructed with a NULL name
will all have the name "compiler_temp" in static storage.

No change Valgrind massif results for a trimmed apitrace of dota2.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>