Shenandoah: Mix-in NULL_PTR in non-strong ShLRBNode's type

Testing found this problem (very rare/hard to reproduce):

# Internal Error (d:/a/jdk/jdk/jdk/src/hotspot/cpu/x86/macroAssembler_x86.cpp:880), pid=6160, tid=5828
# fatal error: DEBUG MESSAGE: unexpected null in intrinsic

I believe this may be caused by non-strong LRB reporting a non-NULL type by passing-through its input's type, even though it may actually return NULL on non-NULL inputs. If this serves as input to an intrinsic, it may lead to elimination of surrounding null-check, and thus end up passing a NULL to the intrinsic even thought it should not.