[X86] Add back the assert from r320830 that was reverted in r320850

Hopefully r320864 has fixed the offending case that failed the assert.

llvm-svn: 320898

lib/Target/X86/MCTargetDesc/X86MCCodeEmitter.cpp

@@ -1130,6 +1130,8 @@ bool X86MCCodeEmitter::emitOpcodePrefix(uint64_t TSFlags, unsigned &CurByte,
       EmitByte(0x40 | REX, CurByte, OS);
       Ret = true;
     }
+  } else {
+    assert(!(TSFlags & X86II::REX_W) && "REX.W requires 64bit mode.");
   }
 
   // 0x0F escape code must be emitted just before the opcode.