RFR (S): JDK-8214027: Reinstate testB_mem_imm pattern in x86_64.ad

[B. Blaser]

Hi,

Please review the following fix for [1] which simply reinstate
'testB_mem_imm' in 'x86_64.ad' as discussed in [2]:

http://cr.openjdk.java.net/~bsrbnd/jdk8214027/webrev.00/

Tier1 (hotspot+jdk+langtools) is OK.

Thanks,
Bernard

[1] https://bugs.openjdk.java.net/browse/JDK-8214027
[2] http://mail.openjdk.java.net/pipermail/hotspot-dev/2018-November/035257.html