Eliminate samples with the same texture coordinates

Adding ReassociatePass before GVN pass ensures the second sample gets
eliminated.

Author: iwwu

IGC/Compiler/CISACodeGen/ShaderCodeGen.cpp

@@ -1411,6 +1411,9 @@ void OptimizeIR(CodeGenContext *const pContext) {
       if (IGC_IS_FLAG_ENABLED(OCLEnableReassociate) && pContext->type == ShaderType::OPENCL_SHADER) {
         mpm.add(createReassociatePass());
       }
+      if (pContext->type == ShaderType::COMPUTE_SHADER) {
+        mpm.add(llvm::createReassociatePass());
+      }
 
       mpm.add(createPromoteConstantStructsPass());