glsl: Don't allocate a name for ir_var_temporary variables

glsl: Don't allocate a name for ir_var_temporary variables

Valgrind massif results for a trimmed apitrace of dota2:

                  n        time(i)         total(B)   useful-heap(B) extra-heap(B)    stacks(B)
Before (32-bit): 74 40,578,719,715       67,762,208       62,263,404     5,498,804            0
After  (32-bit): 52 40,565,579,466       66,359,800       61,187,818     5,171,982            0

Before (64-bit): 74 37,129,541,061       95,195,160       87,369,671     7,825,489            0
After  (64-bit): 76 37,134,691,404       93,271,352       85,900,223     7,371,129            0

A real savings of 1.0MiB on 32-bit and 1.4MiB on 64-bit.

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