Fix for 'not fully instantiated' Crossgen2 issue bucket (#43054)

Fix for 'not fully instantiated' Crossgen2 issue bucket (#43054)

Around September 25 several dozens of Crossgen2 tests started
failing with a runtime error around incomplete instantiation of
GetArrayDataReference. I believe that for generic methods we
should skip the METHOD_ENTRY_DEF_TOKEN shortcut, otherwise we
lose the instantiation information and cause the runtime problem.

I originally thought this may be related to JanK's function
pointer changes but I no longer believe it is the case
(my apologies to Jan for the false accusation). I rather think
that some ambient code change caused a subtle difference in IL
encoding of access to the method that started tripping the
"shortcut" code path.

Thanks

Tomas