[Transforms][SROA] Fixed assertion crash (PR30416)

[kbelochapka]

Summary

Problem:
In certain situations during ALLOCA promotion, the promoteSingleBlockAlloca() function attempts to replace an instruction by the same instruction. This causes the assertion crash described in PR30416.

Solution:
Disable SROA for a Basic Block that can cause circular dependency.

[lebedev.ri]

Test?

[kbelochapka]

In D43544#1014102, @lebedev.ri wrote:

Test?

Added test.

[kbelochapka]

This problem was fixed:

commit 2cd9451d45d2cfd560395c25c12d27d8f6090d1b
Date:   Thu Jun 7 11:09:05 2018 +0000

[Mem2Reg] Avoid replacing load with itself in promoteSingleBlockAlloca.
We do the same thing in rewriteSingleStoreAlloca.

Differential Revision: https://reviews.llvm.org/D47825

[RKSimon]

@kbelochapka Abandon this patch?

[kbelochapka]

Abandoned because of https://reviews.llvm.org/D47825

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);
 
+      // we have a loop, bail out
+      if (LI == ReplVal)
+        return false;
       LI->replaceAllUsesWith(ReplVal);
     }
 
     LI->eraseFromParent();
     LBI.deleteValue(LI);
   }
 
   // Remove the (now dead) stores and alloca.