1) Fix an issue with non-deterministic iteration order in phi_translate

2) Remove some maximal-set computing code that is no longer used.
3) Use a post-order CFG traversal to compute ANTIC_IN instead of a postdom traversal.
This causes the ANTIC_IN calculation to converge much faster.  Thanks to Daniel Berlin for suggesting this.

With this patch, the time to optimize 403.gcc decreased from 17.5s to 7.5s, and Anton's huge
testcase decreased from 62 minutes to 38 seconds.

llvm-svn: 37714