[mlir] ArithToLLVM: fix memref bitcast lowering (#125148)
`arith.bitcast` is allowed on memrefs and such code can actually be
generated by IREE `ConvertBf16ArithToF32Pass`.
`LLVM::detail::vectorOneToOneRewrite` doesn't properly check its types
and will generate bitcast between structs which is illegal.
With the opaque pointers this is a no-op operation for memref so we can
just add type check in `LLVM::detail::vectorOneToOneRewrite` and add a
separate pattern which removes op if converted types are the same.
