Handle critical edges consistently in all SIL stages.

This will allow passes that optimize phis to be run in the mandatory
pipeline. It also simplifies SIL invariants and will unblock
much needed simplification of BlockArgument handling.

Author: atrick

lib/SIL/SILVerifier.cpp

@@ -4530,29 +4530,26 @@ class SILVerifier : public SILVerifierBase<SILVerifier> {
       return true;
     };
 
-    SILModule &M = F->getModule();
     for (auto &BB : *F) {
       TermInst *TI = BB.getTerminator();
       CurInstruction = TI;
 
-      // Check for non-cond_br critical edges in canonical SIL.
-      if (!isa<CondBranchInst>(TI) && M.getStage() == SILStage::Canonical) {
+      // Check for non-cond_br critical edges.
+      auto *CBI = dyn_cast<CondBranchInst>(TI);
+      if (!CBI) {
         for (unsigned Idx = 0, e = BB.getSuccessors().size(); Idx != e; ++Idx) {
           require(!isCriticalEdgePred(TI, Idx),
                   "non cond_br critical edges not allowed");
         }
         continue;
       }
-
       // In ownership qualified SIL, ban critical edges from CondBranchInst that
       // have non-trivial arguments.
+      //
+      // FIXME: it would be far simpler to ban all critical edges in general.
       if (!F->hasQualifiedOwnership())
         continue;
 
-      auto *CBI = dyn_cast<CondBranchInst>(TI);
-      if (!CBI)
-        continue;
-
       if (isCriticalEdgePred(CBI, CondBranchInst::TrueIdx)) {
         require(
             llvm::all_of(CBI->getTrueArgs(),