glsl: Squish ir_variable::max_ifc_array_access and ::state_slots together

glsl: Squish ir_variable::max_ifc_array_access and ::state_slots together

At least one of these pointers must be NULL, and we can determine which
will be NULL by looking at other fields.  Use this information to store
both pointers in the same location.

If anyone can think of a better name for the union than "u", I'm all
ears.

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): 63 40,574,239,515       68,117,280       62,618,607     5,498,673            0
After  (32-bit): 44 40,577,049,140       68,118,608       62,441,063     5,677,545            0

Before (64-bit): 53 37,126,451,468       95,150,256       87,711,304     7,438,952            0
After  (64-bit): 63 37,122,829,194       95,153,008       87,333,600     7,819,408            0

A real savings of 173KiB on 32-bit and 368KiB on 64-bit.

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