IR should have !nofpclass metadata, equivalent to nofpclass attribute

[arsenm]

The IR should define a !nofpclass metadata type, equivalent to the nofpclass attribute. This will just encode an integer equivalent to the attribute's bitmask.

This will be useful to complete the migration to only use byref arguments with amdgpu_kernel. If we stop using direct value arguments, we currently have no equivalent way of representing the nofpclass annotation.