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