AMDGPU: Emit metadata for the hidden_multigrid_sync_arg conditionally

Summary:
  Introduce a new function attribute, amdgpu-no-multigrid-sync-arg, which is default.
We use implicitarg_ptr + offset to check whether the multigrid synchronization
pointer is used. If yes, we remove this attribute and also remove
amdgpu-no-implicitarg-ptr. We generate metadata for the hidden_multigrid_sync_arg
only when the amdgpu-no-multigrid-sync-arg attribute is removed from the function.

Reviewers: arsenm, sameerds, b-sumner and foad

Differential Revision: https://reviews.llvm.org/D123548