[CodeGen] Avoid nullptr arg to CreateStructGEP (NFC)

For now just make the getPointerElementType() explicit.

clang/lib/CodeGen/CGCall.cpp

@@ -3437,7 +3437,8 @@ void CodeGenFunction::EmitFunctionEpilog(const CGFunctionInfo &FI,
       --EI;
       llvm::Value *ArgStruct = &*EI;
       llvm::Value *SRet = Builder.CreateStructGEP(
-          nullptr, ArgStruct, RetAI.getInAllocaFieldIndex());
+          EI->getType()->getPointerElementType(), ArgStruct,
+          RetAI.getInAllocaFieldIndex());
       llvm::Type *Ty =
           cast<llvm::GetElementPtrInst>(SRet)->getResultElementType();
       RV = Builder.CreateAlignedLoad(Ty, SRet, getPointerAlign(), "sret");