Index: lib/Analysis/ScalarEvolutionExpander.cpp
===================================================================
--- lib/Analysis/ScalarEvolutionExpander.cpp
+++ lib/Analysis/ScalarEvolutionExpander.cpp
@@ -1170,7 +1170,9 @@
           continue;
 
       Instruction *TempIncV =
-          cast<Instruction>(PN.getIncomingValueForBlock(LatchBlock));
+          dyn_cast<Instruction>(PN.getIncomingValueForBlock(LatchBlock));
+      if (!TempIncV)
+        continue;
 
       // Check whether we can reuse this PHI node.
       if (LSRMode) {
Index: test/Transforms/LoopStrengthReduce/scev-after-loopinstsimplify.ll
===================================================================
--- /dev/null
+++ test/Transforms/LoopStrengthReduce/scev-after-loopinstsimplify.ll
@@ -0,0 +1,43 @@
+; RUN: opt %s -indvars -loop-instsimplify -loop-reduce
+; We are only checking that there is no crash!
+
+; https://bugs.llvm.org/show_bug.cgi?id=37936
+
+; The problem is as follows:
+; 1. indvars marks %dec as NUW.
+; 2. loop-instsimplify runs instsimplify, which constant-folds %dec to -1
+; 3. loop-reduce tries to do some further modification, but crashes
+;    with an type assertion in cast, because %dec is no longer an Instruction,
+;    even though the SCEV data indicated it was.
+
+; If the runline is split into two, i.e. -indvars -loop-instsimplify first, that
+; stored into a file, and then -loop-reduce is run on that, there is no crash.
+; So it looks like the problem is due to -loop-instsimplify not discarding SCEV.
+
+target datalayout = "n16"
+
+@a = external global i16, align 1
+
+define void @f1() {
+entry:
+  br label %for.cond
+
+for.cond:                                         ; preds = %land.end, %entry
+  %c.0 = phi i16 [ 0, %entry ], [ %dec, %land.end ]
+  br i1 undef, label %for.body, label %for.cond.cleanup
+
+for.cond.cleanup:                                 ; preds = %for.cond
+  ret void
+
+for.body:                                         ; preds = %for.cond
+  %0 = load i16, i16* @a, align 1
+  %cmp = icmp sgt i16 %0, %c.0
+  br i1 %cmp, label %land.rhs, label %land.end
+
+land.rhs:                                         ; preds = %for.body
+  unreachable
+
+land.end:                                         ; preds = %for.body
+  %dec = add nsw i16 %c.0, -1
+  br label %for.cond
+}