Remove OldCmm, convert backends to consume new Cmm
This removes the OldCmm data type and the CmmCvt pass that converts new Cmm to OldCmm. The backends (NCGs, LLVM and C) have all been converted to consume new Cmm. The main difference between the two data types is that conditional branches in new Cmm have both true/false successors, whereas in OldCmm the false case was a fallthrough. To generate slightly better code we occasionally need to invert a conditional to ensure that the branch-not-taken becomes a fallthrough; this was previously done in CmmCvt, and it is now done in CmmContFlowOpt. We could go further and use the Hoopl Block representation for native code, which would mean that we could use Hoopl's postorderDfs and analyses for native code, but for now I've left it as is, using the old ListGraph representation for native code.