BarrierSet has nothing to do with c1's LIRAssembler. They all go to macroAssembler.

eg.change as follows can still build on x86_64.

diff --git a/src/hotspot/cpu/x86/c1_LIRAssembler_x86.cpp b/src/hotspot/cpu/x86/c1_LIRAssembler_x86.cpp
--- a/src/hotspot/cpu/x86/c1_LIRAssembler_x86.cpp
+++ b/src/hotspot/cpu/x86/c1_LIRAssembler_x86.cpp
@@ -32,8 +32,6 @@
 #include "c1/c1_ValueStack.hpp"
 #include "ci/ciArrayKlass.hpp"
 #include "ci/ciInstance.hpp"
-#include "gc/shared/barrierSet.hpp"
-#include "gc/shared/cardTableBarrierSet.hpp"
 #include "gc/shared/collectedHeap.hpp"
 #include "nativeInst_x86.hpp"
 #include "oops/objArrayKlass.hpp"