[Mem2Reg] Avoid replacing load with itself in promoteSingleBlockAlloca.

[fhahn]

Summary

We do the same thing in rewriteSingleStoreAlloca.

Fixes PR37632.

[davide]

It seems reasonable to me.

lib/Transforms/Utils/PromoteMemoryToRegister.cpp

if (I == StoresByIndex.begin()) {
       // Otherwise, there was a store before this load, the load takes its value.
       // Note, if the load was marked as nonnull we don't want to lose that
       // information when we erase it. So we preserve it with an assume.
       Value *ReplVal = std::prev(I)->second->getOperand(0);
       if (AC && LI->getMetadata(LLVMContext::MD_nonnull) &&
           !isKnownNonZero(ReplVal, DL, 0, AC, LI, &DT))
         addAssumeNonNull(AC, LI);
 
+      // If the replacement value is the load, this must occur in unreachable
+      // code.
+      if (ReplVal == LI)
+        ReplVal = UndefValue::get(LI->getType());
+
       LI->replaceAllUsesWith(ReplVal);
     }
 
     LI->eraseFromParent();
     LBI.deleteValue(LI);
   }
 
   // Remove the (now dead) stores and alloca.

test/Transforms/Mem2Reg/pr37632-unreachable-list-of-stores.ll

+; RUN: opt -mem2reg < %s -S | FileCheck %s
+
+
+; CHECK-LABEL: void @patatino()
+; CHECK-NEXT: ret void
+
+; CHECK-LABEL: cantreachme:
+; CHECK-NEXT: %dec = add nsw i32 undef, -1
+; CHECK-NEXT: br label %cantreachme
+
+define void @patatino() {
+  %a = alloca i32, align 4
+  ret void
+cantreachme:
+  %dec = add nsw i32 %tmp, -1
+  store i32 %dec, i32* %a
+  store i32 %tmp, i32* %a
+  %tmp = load i32, i32* %a
+  br label %cantreachme
+}