[LoopDeletion] Separate logic in breakBackedgeIfNotTaken using symboic max trip count [nfc]

As mentioned in D108833, the logic for figuring out if a backedge is dead was somewhat interwoven with the SCEV based logic and the symbolic eval logic. This is my attempt at making the code easier to follow.

Note that this is only NFC after the work done in 29fa37e.  Thanks to Nikita for catching that case.

Differential Revision: https://reviews.llvm.org/D108848

Author: preames

llvm/lib/Transforms/Scalar/LoopDeletion.cpp

@@ -393,19 +393,23 @@ breakBackedgeIfNotTaken(Loop *L, DominatorTree &DT, ScalarEvolution &SE,
   if (!L->getLoopLatch())
     return LoopDeletionResult::Unmodified;
 
-  auto *BTCMax = SE.getConstantMaxBackedgeTakenCount(L);
-  if (!BTCMax->isZero()) {
-    auto *BTC = SE.getBackedgeTakenCount(L);
-    if (!BTC->isZero()) {
-      if (!isa<SCEVCouldNotCompute>(BTC) && SE.isKnownNonZero(BTC))
-        return LoopDeletionResult::Unmodified;
-      if (!canProveExitOnFirstIteration(L, DT, LI))
-        return LoopDeletionResult::Unmodified;
-    }
+  auto *BTC = SE.getSymbolicMaxBackedgeTakenCount(L);
+  if (BTC->isZero()) {
+    // SCEV knows this backedge isn't taken!
+    breakLoopBackedge(L, DT, SE, LI, MSSA);
+    return LoopDeletionResult::Deleted;
   }
 
-  breakLoopBackedge(L, DT, SE, LI, MSSA);
-  return LoopDeletionResult::Deleted;
+  // If SCEV leaves open the possibility of a zero trip count, see if
+  // symbolically evaluating the first iteration lets us prove the backedge
+  // unreachable.
+  if (isa<SCEVCouldNotCompute>(BTC) || !SE.isKnownNonZero(BTC))
+    if (canProveExitOnFirstIteration(L, DT, LI)) {
+      breakLoopBackedge(L, DT, SE, LI, MSSA);
+      return LoopDeletionResult::Deleted;
+    }
+
+  return LoopDeletionResult::Unmodified;
 }
 
 /// Remove a loop if it is dead.