[NFC][Utils] deleteDeadLoop(): add an assert that exit block has some non-PHI instruction

[lebedev.ri]

Summary

If deleteDeadLoop() is called on such a loop, that has "bad" exit block,
one that e.g. has no terminator instruction, the DIBuilder::insertDbgValueIntrinsic()
will be told to insert the Dbg Value Intrinsic after nullptr
(since there is no first non-PHI instruction), which will cause it to not insert
those instructions into any basic block. The instructions will be parent-less,
and IR verifier will complain. It is rather obvious to track down the root cause
when that happens, so let's just assert it never happens.

[vsk]

Thanks, lgtm.

[lebedev.ri]

In D61008#1479450, @vsk wrote:

Thanks, lgtm.

Thank you for the review!

lib/Transforms/Utils/LoopUtils.cpp

for (auto *Block : L->blocks())
     }
 
   // After the loop has been deleted all the values defined and modified
   // inside the loop are going to be unavailable.
   // Since debug values in the loop have been deleted, inserting an undef
   // dbg.value truncates the range of any dbg.value before the loop where the
   // loop used to be. This is particularly important for constant values.
   DIBuilder DIB(*ExitBlock->getModule());
+  Instruction *InsertDbgValueBefore = ExitBlock->getFirstNonPHI();
+  assert(InsertDbgValueBefore &&
+         "There should be a non-PHI instruction in exit block, else these "
+         "instructions will have no parent.");
   for (auto *DVI : DeadDebugInst)
-    DIB.insertDbgValueIntrinsic(
-        UndefValue::get(Builder.getInt32Ty()), DVI->getVariable(),
-        DVI->getExpression(), DVI->getDebugLoc(), ExitBlock->getFirstNonPHI());
+    DIB.insertDbgValueIntrinsic(UndefValue::get(Builder.getInt32Ty()),
+                                DVI->getVariable(), DVI->getExpression(),
+                                DVI->getDebugLoc(), InsertDbgValueBefore);
 
   // Remove the block from the reference counting scheme, so that we can
   // delete it freely later.
   for (auto *Block : L->blocks())
     Block->dropAllReferences();
 
   if (LI) {
     // Erase the instructions and the blocks without having to worry