[LoongArch] Fix a warning

This patch fixes:

  llvm/lib/Target/LoongArch/MCTargetDesc/LoongArchAsmBackend.cpp:389:9:
  error: unused variable 'Layout' [-Werror,-Wunused-variable]

Author: kazutakahirata

llvm/lib/Target/LoongArch/MCTargetDesc/LoongArchAsmBackend.cpp

@@ -402,6 +402,7 @@ bool LoongArchAsmBackend::relaxDwarfCFA(const MCAssembler &Asm,
       Layout.getAssembler().getContext().getAsmInfo()->getMinInstAlignment() ==
           1 &&
       "expected 1-byte alignment");
+  (void)Layout;
   if (Value == 0) {
     WasRelaxed = OldSize != Data.size();
     return true;