Subzero: Fix a potential null-pointer dereference.

src/IceTargetLoweringX86BaseImpl.h

Index: src/IceTargetLoweringX86BaseImpl.h
diff --git a/src/IceTargetLoweringX86BaseImpl.h b/src/IceTargetLoweringX86BaseImpl.h
index 1dcb9a3d865dc95f6c107be00c6894838ad0a7dd..0cd2789c3841d6823415b2f8c5c1798eee5148cb 100644
--- a/src/IceTargetLoweringX86BaseImpl.h
+++ b/src/IceTargetLoweringX86BaseImpl.h
@@ -5327,14 +5327,17 @@ const Inst *AddressOptimizer::matchOffsetIndexOrBase(
         }
       } 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;
       }