Initial checkin of PRE on LLVM. This implementation is still lacking in

Initial checkin of PRE on LLVM.  This implementation is still lacking in
several ways:

 * Load expressions are not PRE'd well.  Alias Analysis should be used to
   get accurate information when computing anticipatibility.
 * The expression collection implementation does not handle PHI nodes properly,
   thus the implementation misses many opportunities to PRE.
 * This code could be sped up quite a bit

Despite these flaws, the code seems to work well, and handles PR's as one
would expect.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@5759 91177308-0d34-0410-b5e6-96231b3b80d8