[llvm] [SandboxIR] Fix base class of FenceInst. Verify instructions when building a BB in debug mode. (PR #108078)

[Jorge Gorbe Moya via llvm-commits]

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@@ -3249,4 +3249,12 @@ void BasicBlock::dumpOS(raw_ostream &OS) const {
     }
   }
 }
+
+void BasicBlock::verify() const {
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slackito wrote:

It is in the same `#ifdef NDEBUG` block as the `dumpOS` method right above it, isn't it?

https://github.com/llvm/llvm-project/pull/108078