When the processor model varies for more recent generations of EC2 instances it is documented here: https://aws.amazon.com/ec2/instance-types/

Scroll down to get to the table that includes Physical Processor, Intel AVX, Intel AVX2, etc.

T2 instances do run on a number of different processor models, therefore it is listed as "Intel Xeon Family." M4 instances run on Intel Xeon E5-2676 v3 (Haswell) or Intel Xeon E5-2686 v4 (Broadwell), though m4.10xlarge only runs on Haswell and m4.16xlarge only runs on Broadwell. Pay close attention to the * in the table for those instances that run on either Haswell or Broadwell.

Generally you will find that the recent generations of EC2 instances for C and R have identical CPUs within a generation.

very good point, this has been my experience as well, particularly for burstable instances such as the T2s