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
