More like 3.0e16 sha-256 computations per second (each Bitcoin hash involves two sha256 calculations), but the HLT idle command is a clear winner.
Assume: HLT idle is 1 clock cycle
Assume: Typical PC is idling at 1 GHz
PC shipments were 350m in 2010. If even 1 billion PCs are on and idle 60% of the day, then we have:
0.6 * 1 billion * 1 GHz * 1 year = 1.9e25 iterations per year
3.0e16 Hz * 1 year = 9.5e23 iterations per year for Bitcoin
Now, sha256x2 is much more complicated than HLT, but the OP asked for iterations.