[LoopFlatten] Fix invalid assertion (PR49571)

The `InductionPHI` is not necessarily the increment instruction, as
demonstrated in pr49571.ll.
This patch removes the assertion and instead bails out from the
`LoopFlatten` pass if that happens.

This fixes https://bugs.llvm.org/show_bug.cgi?id=49571

Reviewed By: SjoerdMeijer

Differential Revision: https://reviews.llvm.org/D99252

llvm/lib/Transforms/Scalar/LoopFlatten.cpp

@@ -176,8 +176,11 @@ static bool findLoopComponents(
   LLVM_DEBUG(dbgs() << "Found limit: "; Limit->dump());
 
   assert(InductionPHI->getNumIncomingValues() == 2);
-  assert(InductionPHI->getIncomingValueForBlock(Latch) == Increment &&
-         "PHI value is not increment inst");
+
+  if (InductionPHI->getIncomingValueForBlock(Latch) != Increment) {
+    LLVM_DEBUG(dbgs() << "PHI value is not increment inst");
+    return false;
+  }
 
   auto *CI = dyn_cast<ConstantInt>(
       InductionPHI->getIncomingValueForBlock(L->getLoopPreheader()));

llvm/test/Transforms/LoopFlatten/pr49571.ll

@@ -0,0 +1,24 @@
+; RUN: opt < %s -S -loop-flatten -verify-loop-info -verify-dom-info -verify-scev -verify | FileCheck %s
+
+; CHECK-LABEL: @main
+
+define dso_local void @main() {
+entry:
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.end, %entry
+  br label %for.body
+
+for.body:                                         ; preds = %for.inc, %for.cond
+  %a.03 = phi i32 [ 0, %for.cond ], [ %inc, %for.inc ]
+  br label %for.inc
+
+for.inc:                                          ; preds = %for.body
+  %0 = add i32 %a.03, 1
+  %cmp = icmp slt i32 %0, 10
+  %inc = add nsw i32 %a.03, 1
+  br i1 %cmp, label %for.body, label %for.end
+
+for.end:                                          ; preds = %for.inc
+  br label %for.cond
+}