[PATCH] Turn add, icmp, select into icmp, select with constants.

[Eli Friedman]

  test68a-c already pass without your patch, and test68f checks that we miscompile the testcase.  Did you really mean to send this to llvm-commits?

  (test68g and test68h are intentionally not transformed to avoid increasing register pressure; it's possible we could improve the heuristic.)

http://llvm-reviews.chandlerc.com/D1633