Change the table datastructure to be a vector<smallvector>, instead of

Change the table datastructure to be a vector<smallvector>, instead of
vector<vector> to avoid allocations.  This speeds up bcwriting of 447.dealII
from 0.8276 to 0.7637s (8.4%).

This concludes this round of proding the bcwriter into submission.  Final
speedup from 24.4s to 0.7637s (32x).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@34142 91177308-0d34-0410-b5e6-96231b3b80d8