add an abbreviation for the string constants opzn, shrinking the constnats

block from:

  Block ID #11 (CONSTANTS_BLOCK):
      Num Instances: 1722
         Total Size: 3.85976e+06b/482470B/120617W
          % of file: 16.7609
       Average Size: 2241.44b/280.18B/70.045W
  Tot/Avg SubBlocks: 0/0
    Tot/Avg Abbrevs: 1/0.00058072
    Tot/Avg Records: 26423/15.3444
      % Abbrev Recs: 69.1746

to:

 Block ID #11 (CONSTANTS_BLOCK):
      Num Instances: 1724
         Total Size: 2.62406e+06b/328008B/82001.9W
          % of file: 12.041
       Average Size: 1522.08b/190.26B/47.5649W
  Tot/Avg SubBlocks: 0/0
    Tot/Avg Abbrevs: 2/0.00116009
    Tot/Avg Records: 26280/15.2436
      % Abbrev Recs: 68.9992

This shrinks kc++ from 2815788 to 2724088 bytes, which means the bitcode
file is now smaller than the bytecode file.

llvm-svn: 36820