generalize reg_{to,is}_pointer_sized() to non-GPR registers

[derekbruening]

Pasting from #6725 (comment) which says for for reg_is_pointer_sized():

The docs say "Returns true iff it refers to a pointer-sized general-purpose register."
And x86 only checks GPRs.

However: reg_to_pointer_sized()'s implementation handles SIMD and I think we should clarify its docs to include expanding SIMD: more like "reg_to_container_sized()" or sthg.

Does that mean we'd want reg_is_pointer_sized() to really be reg_is_largest_containing_size()?

Should we add the new names and deprecate the old, or add the new and keep the old, or just update the docs and the is_ implementation? PR 6725 is implementing is_ for SIMD for aarch64.