[mlir] Modify GPU to SPIR-V conversion to respect spv.interface_var_abi attributes if it exists already.

mlir/lib/Conversion/GPUToSPIRV/ConvertGPUToSPIRV.cpp

 }
 
 LogicalResult GPUFuncOpConversion::matchAndRewrite(
     gpu::GPUFuncOp funcOp, ArrayRef<Value> operands,
     ConversionPatternRewriter &rewriter) const {
   if (!gpu::GPUDialect::isKernel(funcOp))
     return failure();
 
-  // TODO(antiagainst): we are dictating the ABI by ourselves here; it should be
-  // specified outside.
   SmallVector<spirv::InterfaceVarABIAttr, 4> argABI;
   for (auto argIndex : llvm::seq<unsigned>(0, funcOp.getNumArguments())) {
+    // If the ABI is already specified, use it.
+    auto abiAttr = funcOp.getArgAttrOfType<spirv::InterfaceVarABIAttr>(
+        argIndex, spirv::getInterfaceVarABIAttrName());
+    if (abiAttr) {
+      argABI.push_back(abiAttr);
+      continue;
+    }
+    // todo(ravishankarm): Use the "default ABI". Remove this in a follow up
+    // CL. Staging this to make this easy to revert in case of breakages out of
+    // tree.
     Optional<spirv::StorageClass> sc;
     if (funcOp.getArgument(argIndex).getType().isIntOrIndexOrFloat())
       sc = spirv::StorageClass::StorageBuffer;
     argABI.push_back(
         spirv::getInterfaceVarABIAttr(0, argIndex, sc, rewriter.getContext()));
   }
 
   auto entryPointAttr = spirv::lookupEntryPointABI(funcOp);