[WebAssembly][MC] Fix -Wunused-variable in -DLLVM_ENABLE_ASSERTIONS=off builds after r369317

llvm-svn: 369318
diff --git a/llvm/lib/MC/WasmObjectWriter.cpp b/llvm/lib/MC/WasmObjectWriter.cpp
index 89c1d83..cb00241 100644
--- a/llvm/lib/MC/WasmObjectWriter.cpp
+++ b/llvm/lib/MC/WasmObjectWriter.cpp
@@ -426,10 +426,8 @@
                                         const MCFragment *Fragment,
                                         const MCFixup &Fixup, MCValue Target,
                                         uint64_t &FixedValue) {
-  MCAsmBackend &Backend = Asm.getBackend();
-
   // The WebAssembly backend should never generate FKF_IsPCRel fixups
-  assert(!(Backend.getFixupKindInfo(Fixup.getKind()).Flags &
+  assert(!(Asm.getBackend().getFixupKindInfo(Fixup.getKind()).Flags &
            MCFixupKindInfo::FKF_IsPCRel));
 
   const auto &FixupSection = cast<MCSectionWasm>(*Fragment->getParent());