There is an unprotected UseCompressedOops block here:

diff -r 7f22637faf49 src/hotspot/cpu/x86/gc/shenandoah/shenandoahBarrierSetC1_x86.cpp
--- a/src/hotspot/cpu/x86/gc/shenandoah/shenandoahBarrierSetC1_x86.cpp Mon May 06 19:26:35 2019 +0200
+++ b/src/hotspot/cpu/x86/gc/shenandoah/shenandoahBarrierSetC1_x86.cpp Mon May 06 19:26:56 2019 +0200
@@ -46,12 +46,14 @@
   // Apply storeval barrier to newval.
   ShenandoahBarrierSet::assembler()->storeval_barrier(masm->masm(), newval, tmp1);
 
+#ifdef _LP64
   if (UseCompressedOops) {
     __ encode_heap_oop(cmpval);
     __ mov(rscratch1, newval);
     __ encode_heap_oop(rscratch1);
     newval = rscratch1;
   }
+#endif
 
   ShenandoahBarrierSet::assembler()->cmpxchg_oop(masm->masm(), result, Address(addr, 0), cmpval, newval, false, tmp1, tmp2);
 }

x86_32 fails to build because of this.