Fix sampler slot extraction on gather4_po_c when debugging dxbc shader
Fix sampler slot extraction on gather4_po_c when debugging dxbc shader

gather4_po_c has 6 operands and offset of sampler should be 4 instead of 3.

Zorro666/renderdoc,michaelrgb/renderdoc,TurtleRockStudios/renderdoc_public,michaelrgb/renderdoc,etnlGD/renderdoc,etnlGD/renderdoc,baldurk/renderdoc,michaelrgb/renderdoc,cgmb/renderdoc,TurtleRockStudios/renderdoc_public,googlestadia/renderdoc,Zorro666/renderdoc,etnlGD/renderdoc,googlestadia/renderdoc,googlestadia/renderdoc,etnlGD/renderdoc,Zorro666/renderdoc,baldurk/renderdoc,cgmb/renderdoc,TurtleRockStudios/renderdoc_public,Zorro666/renderdoc,TurtleRockStudios/renderdoc_public,etnlGD/renderdoc,cgmb/renderdoc,baldurk/renderdoc,TurtleRockStudios/renderdoc_public,cgmb/renderdoc,michaelrgb/renderdoc,googlestadia/renderdoc,baldurk/renderdoc,TurtleRockStudios/renderdoc_public,Zorro666/renderdoc,etnlGD/renderdoc,baldurk/renderdoc,michaelrgb/renderdoc,baldurk/renderdoc,cgmb/renderdoc,googlestadia/renderdoc,Zorro666/renderdoc,googlestadia/renderdoc