Index: lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
===================================================================
--- lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
+++ lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
@@ -2006,9 +2006,17 @@
       LPM.markLoopAsDeleted(*L);
   };
 
+  assert(!L->isInvalid() && "Unswitching invalid loop?");
   bool Changed =
       unswitchLoop(*L, DT, LI, AC, TTI, NonTrivial, UnswitchCB);
 
+  // We might have done structural changes to a CFG that may break cached
+  // invariants in SCEV for this and any outer loop.
+  // FIXME: Is it OK that unswitching produces invalid loops?!
+  if (Changed && !L->isInvalid())
+    if (auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>())
+      SEWP->getSE().forgetTopmostLoop(L);
+
   // If anything was unswitched, also clear any cached information about this
   // loop.
   LPM.deleteSimpleAnalysisLoop(L);
Index: test/Transforms/SimpleLoopUnswitch/invalidate_scev.ll
===================================================================
--- test/Transforms/SimpleLoopUnswitch/invalidate_scev.ll
+++ test/Transforms/SimpleLoopUnswitch/invalidate_scev.ll
@@ -0,0 +1,34 @@
+; RUN: opt -iv-users -simple-loop-unswitch -indvars -S < %s | FileCheck %s
+; PR37651
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+
+; Make sure that SCEV gets invalidated properly after simple unswitching.
+define void @f1() {
+
+; CHECK-LABEL: @f1
+
+vector.ph:
+  br label %bb3
+
+bb3:                                              ; preds = %bb13, %vector.ph
+  %_tmp5329 = phi i8 [ %_tmp54, %bb13 ], [ undef, %vector.ph ]
+  %_tmp40 = icmp eq i8 undef, 0
+  br label %bb11
+
+bb11:                                             ; preds = %bb12, %bb3
+  br i1 %_tmp40, label %bb12, label %bb13
+
+bb12:                                             ; preds = %bb11
+  br i1 false, label %bb11, label %bb13
+
+bb13:                                             ; preds = %bb12, %bb11
+  %_tmp54 = add nsw i8 %_tmp5329, 1
+  %_tmp57 = icmp eq i8 %_tmp54, 0
+  br i1 %_tmp57, label %bb14.loopexit, label %bb3
+
+bb14.loopexit:                                    ; preds = %bb13
+  ret void
+}