During execution "compile" command information about all removed registers priniting to the console:
Information: The register 'dsample_out_reg[32]' is a constant and will be removed. (OPT-1206)
Information: The register 'dsample_out_reg[33]' is a constant and will be removed. (OPT-1206)
Information: The register 'dsample_out_reg[34]' is a constant and will be removed. (OPT-1206)
By default DC will give a (OPT-1206) warning for all the registers that are removed while optimization.
However if you have certain variables such as compile_seqmap_propagate_high_effort set as false in your synthesis script, then DC will not perform this optimization and hence this message wont appear.
Ya i have tried giving check design but it has only issues such as some nets, ports are connected to ground, multilple pins,ports but doesnt give any info regarding the removed reg.