glsl: Replace ir_variable::warn_extension pointer with an 8-bit index

glsl: Replace ir_variable::warn_extension pointer with an 8-bit index

Also move the new warn_extension_index into ir_variable::data.  This
enables slightly better packing.

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): 82 40,580,040,531       68,488,992       62,973,695     5,515,297            0
After  (32-bit): 73 40,580,476,304       68,488,400       62,796,151     5,692,249            0

Before (64-bit): 65 37,124,013,542       95,892,768       88,466,712     7,426,056            0
After  (64-bit): 71 37,124,890,613       95,889,584       88,089,008     7,800,576            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>