X86: Fix bug in GenerateVarHandleAccessModeCheck

The condition of taking the slow path for VarHandle access mode check
was reversed.

Solving this bug revealed another problem: in the case of an instance
field, the object might be null. The null check is now done for the
object.

Test: art/test.py --host --32 -r -t 712-varhandle-invocations
Bug: 65872996
Change-Id: I8d047cc0ecba3d05178ea67945afe762c545c27e
1 file changed