shave another 150K off of kc++, by using a 7-bit encoding for BB names

shave another 150K off of kc++, by using a 7-bit encoding for BB names
where we can.  This shrinks kc++'s down to 3368K, with a VST record of:

  Block ID #14 (VALUE_SYMTAB):
      Num Instances: 2345
         Total Size: 1.29508e+07b/1.61885e+06B/404713W
       Average Size: 5522.73b/690.342B/172.585W
          % of file: 48.0645
  Tot/Avg SubBlocks: 0/0
    Tot/Avg Abbrevs: 7035/3
    Tot/Avg Records: 120924/51.5667
      % Abbrev Recs: 100

Isn't it nice to be able to optimizer bc size without touching the reader? :)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@36759 91177308-0d34-0410-b5e6-96231b3b80d8