Index: lib/Analysis/ScalarEvolutionExpander.cpp
===================================================================
--- lib/Analysis/ScalarEvolutionExpander.cpp
+++ lib/Analysis/ScalarEvolutionExpander.cpp
@@ -1820,6 +1820,7 @@
                                                    IsomorphicInc->getType());
       if (OrigInc != IsomorphicInc
           && TruncExpr == SE.getSCEV(IsomorphicInc)
+          && SE.LI.replacementPreservesLCSSAForm(IsomorphicInc, OrigInc)
           && ((isa<PHINode>(OrigInc) && isa<PHINode>(IsomorphicInc))
               || hoistIVInc(OrigInc, IsomorphicInc))) {
         DEBUG_WITH_TYPE(DebugType, dbgs()
Index: test/Transforms/IndVarSimplify/pr26974.ll
===================================================================
--- /dev/null
+++ test/Transforms/IndVarSimplify/pr26974.ll
@@ -0,0 +1,60 @@
+; RUN: opt -indvars  -S < %s | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+; indvars will try to replace %b.0.lcssa with %t.1. If it does this,
+; it will break LCSSA.
+
+@c = external global i32, align 4
+
+; CHECK-LABEL: @fn1
+define void @fn1() {
+entry:
+  br label %for.body
+
+for.cond1.preheader:                              ; preds = %for.body
+  %0 = load i32, i32* @c, align 4
+  br i1 undef, label %for.cond1.us.preheader, label %for.cond1
+
+for.cond1.us.preheader:                           ; preds = %for.cond1.preheader
+  br label %for.cond1.us
+
+for.cond1.us:                                     ; preds = %for.cond1.us, %for.cond1.us.preheader
+  br label %for.cond1.us
+
+for.body:                                         ; preds = %for.body, %entry
+  br i1 undef, label %for.body, label %for.cond1.preheader
+
+for.cond1:                                        ; preds = %for.cond1.preheader
+  br i1 true, label %for.body9.lr.ph, label %for.cond13.preheader
+
+for.body9.lr.ph:                                  ; preds = %for.cond1
+  br i1 undef, label %for.body9.us.preheader, label %for.body9
+
+for.body9.us.preheader:                           ; preds = %for.body9.lr.ph
+  br label %for.body9.us
+
+for.body9.us:                                     ; preds = %for.body9.us, %for.body9.us.preheader
+  br label %for.body9.us
+
+for.cond13.preheader:                             ; preds = %for.body9, %for.cond1
+  %b.0.lcssa = phi i32 [ %0, %for.body9 ], [ 0, %for.cond1 ]
+  br label %for.cond13
+
+for.body9:                                        ; preds = %for.body9.lr.ph
+  br label %for.cond13.preheader
+
+for.cond13:                                       ; preds = %for.cond13, %for.cond13.preheader
+  %d.1 = phi i32 [ %t.1, %for.cond13 ], [ %0, %for.cond13.preheader ]
+  %t.1 = phi i32 [ %b.0.lcssa, %for.cond13 ], [ %0, %for.cond13.preheader ]
+  br i1 undef, label %for.cond18.preheader, label %for.cond13
+
+for.cond18.preheader:                             ; preds = %for.cond13
+  br label %for.cond18
+
+for.cond18:                                       ; preds = %for.cond18, %for.cond18.preheader
+  %b.1 = phi i32 [ %xor, %for.cond18 ], [ %b.0.lcssa, %for.cond18.preheader ]
+  %add = add nsw i32 %b.1, %d.1
+  %xor = xor i32 %add, %b.1
+  br label %for.cond18
+}