nak: a few SM50 texture op fixes
Tested against dEQP-VK.glsl.texture_functions.*
:
Test run totals:
Passed: 1017/1464 (69.5%)
Failed: 0/1464 (0.0%)
Not supported: 447/1464 (30.5%)
Warnings: 0/1464 (0.0%)
Waived: 0/1464 (0.0%)