CompactStrings intrinsics should use ArrayCopyNode

LibraryCallKit::inline_string_toBytesU(), LibraryCallKit::inline_string_getCharsU() and PhaseStringOpts::arraycopy() use direct calls to array copy stub. Instead an ArrayCopyNode should be emitted. Currently, this does not work because an ArrayCopyNode requires

  // (2) src and dest arrays must have elements of the same BasicType

Since for ..toBytesU() and ..getCharsU() we are copying from char[] to byte[] or vice-versa, we cannot use ArrayCopyNode without modifying the macro expansion. The same applies to PhaseStringOpts::arraycopy() where we are copying between two byte arrays because the ArrayCopyNode does not "know" that we can always copy chars since the offsets are guaranteed to be char aligned.