Subzero: Clean up some uses of *_cast<>.

src/IceTargetLoweringX8664.cpp

Index: src/IceTargetLoweringX8664.cpp
diff --git a/src/IceTargetLoweringX8664.cpp b/src/IceTargetLoweringX8664.cpp
index 3befd98cb48cfc2ddaf3e0d996b5c77d2c93b037..9b99e81c239dabe208ed3279bb0a94e35316ef6f 100644
--- a/src/IceTargetLoweringX8664.cpp
+++ b/src/IceTargetLoweringX8664.cpp
@@ -412,7 +412,7 @@ Traits::X86OperandMem *TargetX8664::_sandbox_mem_reference(X86OperandMem *Mem) {
   if (Offset != nullptr) {
     if (const auto *CR = llvm::dyn_cast<ConstantRelocatable>(Offset)) {
       NeedsLea = CR->getName() != "" || CR->getOffset() < 0;
-    } else if (const auto *Imm = llvm::cast<ConstantInteger32>(Offset)) {
+    } else if (const auto *Imm = llvm::dyn_cast<ConstantInteger32>(Offset)) {
       NeedsLea = Imm->getValue() < 0;
     } else {
       llvm::report_fatal_error("Unexpected Offset type.");