Minor performance note about IdInfo.
Minor performance note about IdInfo.

Signed-off-by: Edward Z. Yang <dbd597f5635f432486c5d365e9bb585b3eaa1853@cs.stanford.edu>

shlevy/ghc,olsner/ghc,snoyberg/ghc,sdiehl/ghc,sdiehl/ghc,snoyberg/ghc,mettekou/ghc,olsner/ghc,sgillespie/ghc,sgillespie/ghc,mettekou/ghc,ezyang/ghc,snoyberg/ghc,mettekou/ghc,olsner/ghc,mettekou/ghc,mettekou/ghc,sgillespie/ghc,vTurbine/ghc,snoyberg/ghc,sdiehl/ghc,ezyang/ghc,vTurbine/ghc,snoyberg/ghc,sgillespie/ghc,ezyang/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,sgillespie/ghc,olsner/ghc,sdiehl/ghc,snoyberg/ghc,vTurbine/ghc,ezyang/ghc,ezyang/ghc,shlevy/ghc,mettekou/ghc,olsner/ghc,vTurbine/ghc,snoyberg/ghc,mettekou/ghc,ezyang/ghc,olsner/ghc,olsner/ghc,vTurbine/ghc,vTurbine/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,vTurbine/ghc,sgillespie/ghc,sdiehl/ghc,shlevy/ghc,sgillespie/ghc,shlevy/ghc