SNMP on IOS XR is not documented at all right now in my opinion. Of course IOS XR supports standard base of MIB’s but if you want to monitor something platform specific it’s hard to guess which MIB you should use. SNMP Object Navigator on cisco.com does not know yet, there is such think as IOS XR or NX-OS.
I got this information from Cisco representative and I’m putting it here so you can use this too. I had to implement CPU usage monitoring on IOS XR for my customer, but unfortunately standard MIB’s were not working. On IOS XR you have to use cpmCPUTotalTable defined in CISCO-PROCESS-MIB

  cpmCPUTotalIndex          .1.3.6.1.4.1.9.9.109.1.1.1.1.1
  cpmCPUTotalPhysicalIndex  .1.3.6.1.4.1.9.9.109.1.1.1.1.2
  cpmCPUTotal1minRev        .1.3.6.1.4.1.9.9.109.1.1.1.1.7
  cpmCPUTotal5minRev        .1.3.6.1.4.1.9.9.109.1.1.1.1.8

cpmCPUTotalIndex is a unique value to identify each CPU but this is not fixed value. Then you need to check an entity with cpmCPUTotalPhysicalIndex as entPhysicalIndex.

Update: Same MIBs can be used to monitor supervisor usage on Nexus 7000