Add register size info back to MCRegisterClass

This patch addresses the removal of register size information done in
commit c8b782ce.

Without this change, there is no viable option to get register size
information outside libTarget. We need this information to run
analysis that know the register size from the MC layer, used by
BOLT.

Discussion D50285 and D47199.

Reviewed By: kparzysz

Differential Revision: https://reviews.llvm.org/D97891