[mlir:GPU] Replace reference to LLVMFuncOp with FuncOpInterface

The GPU dialect currently contains an explicit reference to  LLVMFuncOp
during verification to handle the situation where the kernel has already been
converted. This commit changes that reference to instead use FunctionOpInterface,
which has two main benefits:

* It allows for removing an otherwise unnecessary dependency on the LLVM dialect
* It removes hardcoded assumptions about the lowering path and use of the GPU dialect

Differential Revision: https://reviews.llvm.org/D118172