[MLIR][XeGPU] fix load/store/prefetch op offset verifier (#166137)
The verifier of `xegpu.{load/store/prefetch}_nd` op fails if `offset` a
mix of static and dynamic values, e.g. `offset = [0, %c0]`. In this case
the length of dynamic offsets is 1 and the check `offsetSize !=
tDescRank` (=2) fails. Instead, we should check the length of
`getMixedOffsets()`.