[mlir][spirv] Only attach struct offset for required storage classes

Per the SPIR-V spec "2.16.2. Validation Rules for Shader Capabilities":

  Composite objects in the StorageBuffer, PhysicalStorageBuffer,
  Uniform, and PushConstant Storage Classes must be explicitly
  laid out.

For other cases we don't need to attach the struct offsets.

Reviewed By: hanchung

Differential Revision: https://reviews.llvm.org/D100386