[TableGen] Ignore inaccessible memory when checking pattern flags (#90061)

In the AMDGPU backend we have some cases where we'd like to mark an
intrinsic as IntrInaccessibleMemOnly to model dependencies, but the
corresponding MachineInstrs use uses/defs of a special physical register
to express the same thing. In this case TableGen would complain:

  Pattern doesn't match mayLoad/mayStore = 0

but the error is not useful.