[scan-build] fix warnings emitted on LLVM IR code base

[apelete]

Summary

Fix "Logic error" warnings of the type "Called C++ object pointer is
null" reported by Clang Static Analyzer on the following file:

• lib/IR/Verifier.cpp.

Signed-off-by: Apelete Seketeli <[e-mail]>

lib/IR/Verifier.cpp

for (auto &I : BB) {
       DISubprogram *SP = Scope ? Scope->getSubprogram() : nullptr;
 
       // Scope and SP could be the same MDNode and we don't want to skip
       // validation in that case
       if (SP && ((Scope != SP) && !Seen.insert(SP).second))
         continue;
 
       // FIXME: Once N is canonical, check "SP == &N".
-      Assert(SP->describes(&F),
+      Assert(SP && SP->describes(&F),
              "!dbg attachment points at wrong subprogram for function", N, &F,
              &I, DL, Scope, SP);
     }
 }
 
 // verifyBasicBlock - Verify that a basic block is well formed...
 //
 void Verifier::visitBasicBlock(BasicBlock &BB) {