[DFAJumpThreading] Fix possible null dereference (#157461)

Fixes #157450

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Co-authored-by: Nikita Popov <[email protected]>

Author: apach301

llvm/lib/Transforms/Scalar/DFAJumpThreading.cpp

@@ -521,7 +521,7 @@ struct MainSwitch {
 
     Instruction *SIUse = dyn_cast<Instruction>(SI->user_back());
     // The use of the select inst should be either a phi or another select.
-    if (!SIUse && !(isa<PHINode>(SIUse) || isa<SelectInst>(SIUse)))
+    if (!SIUse || !(isa<PHINode>(SIUse) || isa<SelectInst>(SIUse)))
       return false;
 
     BasicBlock *SIBB = SI->getParent();