[MemorySSA] Check for unreachable blocks when getting last definition.

If a single predecessor is found, still check if the block is
unreachable. The test that found this had a self loop unreachable block.
Resolves PR43493.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@373383 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/MemorySSAUpdater.cpp

@@ -839,6 +839,9 @@ void MemorySSAUpdater::applyInsertUpdates(ArrayRef<CFGUpdate> Updates,
       } else {
         // Single predecessor, BB cannot be dead. GetLastDef of Pred.
         assert(Count == 1 && Pred && "Single predecessor expected.");
+        // BB can be unreachable though, return LoE if that is the case.
+        if (!DT.getNode(BB))
+          return MSSA->getLiveOnEntryDef();
         BB = Pred;
       }
     };

test/Analysis/MemorySSA/pr43493.ll

@@ -0,0 +1,27 @@
+; RUN: opt -enable-mssa-loop-dependency=true -loop-rotate -verify-memoryssa -S %s | FileCheck %s
+; REQUIRES: asserts
+
+; CHECK-LABEL: @func_35()
+define void @func_35() {
+entry:
+  br i1 undef, label %for.cond1704.preheader, label %return
+
+for.cond1704.preheader:                           ; preds = %entry
+  br label %for.cond1704
+
+for.cond1704:                                     ; preds = %for.cond1704.preheader, %for.body1707
+  br i1 false, label %for.body1707, label %return.loopexit
+
+for.body1707:                                     ; preds = %for.cond1704
+  store i32 1712, i32* undef, align 1
+  br label %for.cond1704
+
+for.body1102:                                     ; preds = %for.body1102
+  br i1 undef, label %for.body1102, label %return
+
+return.loopexit:                                  ; preds = %for.cond1704
+  br label %return
+
+return:                                           ; preds = %return.loopexit, %for.body1102, %entry
+  ret void
+}