In OldCmm, the false case of a conditional was a fallthrough. In Cmm,
conditionals have both true and false successors. When we convert Cmm to LLVM,
we now first re-order Cmm blocks so that the false successor of a conditional
occurs next in the list of basic blocks, i.e., it is a fallthrough, just like it
(necessarily) did in OldCmm. Surprisingly, this can make a big performance
difference.