RFR: fix aarch64 membar elision with shenandoah

[Roland Westrelin]

> The test is used to verify that we do not emit barriers when particular types of barriers are
> disabled: there are asserts all over runtime, C1 and C2 code to test these. Over time, it started to
> be the test to make sure that arguments/heuristics code verifies and drops the incompatible
> combination of flags.

Can we simply skip barrier elision in case of a strange combination of
options? It's a pain to fix that code.

Roland.