[LSR] Preserve MSSA when using SplitCriticalEdge.

LSR claims to MemorySSA, but we also have to make sure it is preserved
when splitting critical edges. This can be done by passing MSSAU to
SplitCriticalEdge.

Fixes PR47557.

lib/Transforms/Scalar/LoopStrengthReduce.cpp

@@ -5382,10 +5382,11 @@ void LSRInstance::RewriteForPHI(
           // Split the critical edge.
           BasicBlock *NewBB = nullptr;
           if (!Parent->isLandingPad()) {
-            NewBB = SplitCriticalEdge(BB, Parent,
-                                      CriticalEdgeSplittingOptions(&DT, &LI)
-                                          .setMergeIdenticalEdges()
-                                          .setKeepOneInputPHIs());
+            NewBB =
+                SplitCriticalEdge(BB, Parent,
+                                  CriticalEdgeSplittingOptions(&DT, &LI, MSSAU)
+                                      .setMergeIdenticalEdges()
+                                      .setKeepOneInputPHIs());
           } else {
             SmallVector<BasicBlock*, 2> NewBBs;
             SplitLandingPadPredecessors(Parent, BB, "", "", NewBBs, &DT, &LI);

test/Analysis/MemorySSA/update-remove-dead-blocks.ll

@@ -0,0 +1,46 @@
+; RUN: opt -loop-unswitch -loop-reduce -loop-simplifycfg -verify-memoryssa -S %s | FileCheck %s
+
+; TODO: also run with NPM, but currently LSR does not preserve LCSSA, causing a verification failure on the test.
+;   opt -passes='loop-mssa(unswitch<nontrivial>,loop-reduce,simplify-cfg)' -verify-memoryssa -S %s | FileCheck %s
+
+; Test case for PR47557.
+
+; REQUIRES: x86-registered-target
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@a = external global i32, align 4
+@c = external global [1 x i32], align 4
+
+define i32* @test() {
+; CHECK-LABEL: @test
+;
+entry:                                                ; preds = %entry
+  br label %for.cond
+
+for.cond:                                         ; preds = %cleanup, %entry
+  %storemerge = phi i64 [ 0, %entry ], [ %inc7, %cleanup ]
+  br label %for.cond2.1
+
+for.body3:                                        ; preds = %for.cond2.2, %for.cond2.1
+  %arrayidx = getelementptr inbounds [1 x i32], [1 x i32]* @c, i64 0, i64 %storemerge
+  ret i32* %arrayidx
+
+cleanup:                                          ; preds = %for.end5, %if.then
+  %inc7 = add nsw i64 %storemerge, 1
+  br label %for.cond
+
+for.cond2.1:                                      ; preds = %for.cond
+  br i1 true, label %for.inc.1, label %for.body3
+
+for.inc.1:                                        ; preds = %for.end.1
+  br i1 false, label %for.body.2, label %cleanup
+
+for.body.2:                                       ; preds = %for.inc.1
+  store i32 0, i32* @a, align 4
+  br label %for.cond2.2
+
+for.cond2.2:                                      ; preds = %for.body.2
+  br i1 true, label %cleanup, label %for.body3
+}