Remove CHECK that GetThis() must always work.

Ensures GetVReg of a reference on compiled code checks that the
value is indeed a reference from the stack maps.

bug: 116683601
bug: 117452149
Test: 686-get-this
Change-Id: I3a370c45786a4892c7888491bec6d5ae64672f6c
diff --git a/test/knownfailures.json b/test/knownfailures.json
index e27a4d6..db06c4e 100644
--- a/test/knownfailures.json
+++ b/test/knownfailures.json
@@ -947,6 +947,7 @@
           "676-resolve-field-type",
           "685-deoptimizeable",
           "685-shifts",
+          "686-get-this",
           "706-checker-scheduler",
           "707-checker-invalid-profile",
           "714-invoke-custom-lambda-metafactory",