Simplify some uses of map/set's find() method to use count() instead.
Simplify code in SafepointIRVerifier::runOnFunction():

Avoid construction of extra set "nowvalid".  Move construction of
"invalid" set to earlier in this function and remove valid Values from
it.
Simplify code in SafepointMachineVerifier::runOnMachineFunction():

Avoid construction of extra set "validByDef".  Move construction of
"invalid" set to earlier in this function and remove valid registers
from it.