[SimplifyCFG] Use a more elegant solution than r261731

The cleanupret instruction has an invariant that it's 'from' operand be
a cleanuppad.  This invariant was violated when we removed a dead block
which removed a cleanuppad leaving behind a cleanupret with an undef
'from' operand.

This was solved in r261731 by staving off the removal of the dead block
to a later pass.

However, it occured to me that we do not need to do this.
Instead, we can simply avoid processing the cleanupret if it has an
undef 'from' operand because we know that it will be removed soon.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@261754 91177308-0d34-0410-b5e6-96231b3b80d8

Author: majnemer

lib/Transforms/Utils/SimplifyCFG.cpp

@@ -3521,6 +3521,12 @@ static bool mergeCleanupPad(CleanupReturnInst *RI) {
 }
 
 bool SimplifyCFGOpt::SimplifyCleanupReturn(CleanupReturnInst *RI) {
+  // It is possible to transiantly have an undef cleanuppad operand because we
+  // have deleted some, but not all, dead blocks.
+  // Eventually, this block will be deleted.
+  if (isa<UndefValue>(RI->getOperand(0)))
+    return false;
+
   if (removeEmptyCleanup(RI))
     return true;
 
@@ -5278,17 +5284,9 @@ bool SimplifyCFGOpt::run(BasicBlock *BB) {
   if ((pred_empty(BB) &&
        BB != &BB->getParent()->getEntryBlock()) ||
       BB->getSinglePredecessor() == BB) {
-    // Get the block mostly empty.
-    Changed |= removeAllNonTerminatorAndEHPadInstructions(BB) > 0;
-    // Now, verify that we succeeded getting the block empty.
-    // This will not be the case if this unreachable BB creates a token which is
-    // consumed by other unreachable blocks.
-    Instruction *FirstNonPHI = BB->getFirstNonPHI();
-    if (isa<TerminatorInst>(FirstNonPHI) && FirstNonPHI->use_empty()) {
-      DEBUG(dbgs() << "Removing BB: \n" << *BB);
-      DeleteDeadBlock(BB);
-      return true;
-    }
+    DEBUG(dbgs() << "Removing BB: \n" << *BB);
+    DeleteDeadBlock(BB);
+    return true;
   }
 
   // Check to see if we can constant propagate this terminator instruction