AArch64: compiler/c2/aarch64/TestVolatilesG1.java fails with "Missing expected output membar_volatile..."

Reliably reproduces on AArch64 and current jdk/jdk:

$ CONF=linux-aarch64-server-fastdebug make images run-test TEST=compiler/c2/aarch64/TestVolatilesG1.java
...
TEST RESULT: Failed. Execution failed: `main' threw exception: java.lang.RuntimeException: Missing expected output membar_volatile \(elided\)!

Bisection shows this is a guilty commit:

changeset: 55898:b5ab9a71aa95
user: manc
date: Mon Aug 05 13:18:31 2019 -0700
summary: 8225776: Optimize branch frequency of G1's write post-barrier in C2