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.

victor-moya/renderdoc,victor-moya/renderdoc,victor-moya/renderdoc,victor-moya/renderdoc,victor-moya/renderdoc,victor-moya/renderdoc