Subzero: Fix a warning (promoted to an error) under NOASSERT=1 build.

BUG= none
R=kschimpf@google.com

Review URL: https://codereview.chromium.org/951283002
diff --git a/src/IceInstX8632.cpp b/src/IceInstX8632.cpp
index 815b617..622c9ed 100644
--- a/src/IceInstX8632.cpp
+++ b/src/IceInstX8632.cpp
@@ -480,6 +480,7 @@
       llvm::report_fatal_error("Assembler can't jmp to memory operand");
     }
   } else if (const auto Mem = llvm::dyn_cast<OperandX8632Mem>(Target)) {
+    (void)Mem;
     assert(Mem->getSegmentRegister() == OperandX8632Mem::DefaultSegment);
     llvm::report_fatal_error("Assembler can't jmp to memory operand");
   } else if (const auto CR = llvm::dyn_cast<ConstantRelocatable>(Target)) {