Prevent invalid cast (#2987)

Not all SPIRVValue are SPIRVInstruction. For example, a SPIRVType is a SPIRVValue but not a SPIRVInstruction, so the cast is undefined behavior. Prevent invalid casts by checking if SV is indeed a SPIRVInstruction before doing the cast.

Original commit:
KhronosGroup/SPIRV-LLVM-Translator@cadedfbbbf8cf1c

Author: maarquitos14

llvm-spirv/lib/SPIRV/SPIRVToLLVMDbgTran.cpp

@@ -159,8 +159,10 @@ void SPIRVToLLVMDbgTran::transDbgInfo(const SPIRVValue *SV, Value *V) {
     return;
 
   if (Instruction *I = dyn_cast<Instruction>(V)) {
-    const SPIRVInstruction *SI = static_cast<const SPIRVInstruction *>(SV);
-    I->setDebugLoc(transDebugScope(SI));
+    if (SV->isInst()) {
+      const SPIRVInstruction *SI = static_cast<const SPIRVInstruction *>(SV);
+      I->setDebugLoc(transDebugScope(SI));
+    }
   }
 }