MFC r272713:

Since introducing the extra mapping in r250103 (head) for architectural performance
events we have actually counted 'Branch Instruction Retired' when people
asked for 'Unhalted core cycles' using the 'unhalted-core-cycles' event mask
mnemonic.

Reviewed by: jimharris
Discussed with: gnn, rwatson
Sponsored by: DARPA/AFRL