Three improvements:

  1. Check for revisiting a block before checking domination, which is faster.
  2. If the stored value isn't an instruction, we don't have to check for domination.
  3. If we have a value used in the same block more than once, make sure to remove the
     block from the UsingBlocks vector.  Not doing so forces us to go through the slow
     path for the alloca.

The combination of these improvements increases the number of allocas on the fastpath
from 8935 to 8982 on PR1432.  This speeds it up from 2.90s to 2.20s (31%)

llvm-svn: 40811