Subzero: Fix a potential null-pointer dereference.

src/IceTargetLoweringX86BaseImpl.h

 //===- subzero/src/IceTargetLoweringX86BaseImpl.h - x86 lowering -*- C++ -*-==//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 5309 matching lines @@
         return nullptr;
 
       SizeT ZeroesAvailable = 0;
       if (VarDef->getOp() == InstArithmetic::Shl) {
         if (auto *ConstInt =
                 llvm::dyn_cast<ConstantInteger32>(VarDef->getSrc(1))) {
           ZeroesAvailable = ConstInt->getValue();
         }
       } else if (VarDef->getOp() == InstArithmetic::Mul) {
         SizeT PowerOfTwo = 0;
-        ConstantInteger32 *MultConst =
-            llvm::dyn_cast<ConstantInteger32>(VarDef->getSrc(0));
-        if (llvm::isPowerOf2_32(MultConst->getValue())) {
-          PowerOfTwo += MultConst->getValue();
+        if (auto *MultConst =
+                llvm::dyn_cast<ConstantInteger32>(VarDef->getSrc(0))) {
+          if (llvm::isPowerOf2_32(MultConst->getValue())) {
+            PowerOfTwo += MultConst->getValue();
+          }
         }
-        MultConst = llvm::dyn_cast<ConstantInteger32>(VarDef->getSrc(1));
-        if (llvm::isPowerOf2_32(MultConst->getValue())) {
-          PowerOfTwo += MultConst->getValue();
+        if (auto *MultConst =
+                llvm::dyn_cast<ConstantInteger32>(VarDef->getSrc(1))) {
+          if (llvm::isPowerOf2_32(MultConst->getValue())) {
+            PowerOfTwo += MultConst->getValue();
+          }
         }
         ZeroesAvailable = llvm::Log2_32(PowerOfTwo) + 1;
       }
       SizeT ZeroesNeeded = llvm::Log2_32(Const->getValue()) + 1;
       if (ZeroesNeeded == 0 || ZeroesNeeded > ZeroesAvailable)
         return nullptr;
       IsAdd = true; // treat it as an add if the above conditions hold
     } else {
       IsAdd = ArithInst->getOp() == InstArithmetic::Add;
     }
@@ skipping 2680 matching lines @@
         emitGlobal(*Var, SectionSuffix);
       }
     }
   } break;
   }
 }
 } // end of namespace X86NAMESPACE
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICETARGETLOWERINGX86BASEIMPL_H