[ConstraintElimination] Support BBs with single successor in CanAdd.

If BB has a single successor, conditions can be added safely.

(cherry-picked from f473d4a)

Author: fhahn

llvm/lib/Transforms/Scalar/ConstraintElimination.cpp

@@ -275,6 +275,10 @@ static bool eliminateConstraints(Function &F, DominatorTree &DT) {
     // Succ (e.g. the case when adding a condition from a pre-header to a loop
     // header).
     auto CanAdd = [&BB, &DT](BasicBlock *Succ) {
+      if (BB.getSingleSuccessor()) {
+        assert(BB.getSingleSuccessor() == Succ);
+        return true;
+      }
       return any_of(successors(&BB),
                     [Succ](const BasicBlock *S) { return S != Succ; }) &&
              all_of(predecessors(Succ), [&BB, &DT, Succ](BasicBlock *Pred) {

llvm/test/Transforms/ConstraintElimination/assumes.ll

@@ -164,7 +164,7 @@ define i1 @assume_does_not_dominates_successor_with_may_unwind_call_before_assum
 ; CHECK-NEXT:    br label [[EXIT]]
 ; CHECK:       exit:
 ; CHECK-NEXT:    [[C_2:%.*]] = icmp eq i16 [[A]], 0
-; CHECK-NEXT:    ret i1 [[C_2]]
+; CHECK-NEXT:    ret i1 true
 ;
 entry:
   br i1 %i.0, label %exit, label %if.then
@@ -189,7 +189,7 @@ define i1 @assume_dominates_successor_with_may_unwind_call_before_assume_uncond_
 ; CHECK-NEXT:    br label [[EXIT:%.*]]
 ; CHECK:       exit:
 ; CHECK-NEXT:    [[C_2:%.*]] = icmp eq i16 [[A]], 0
-; CHECK-NEXT:    ret i1 [[C_2]]
+; CHECK-NEXT:    ret i1 true
 ;
 entry:
   call void @may_unwind()