8176100: [REDO][REDO] G1 Needs pre barrier on dereference of weak JNI handles
Reviewed-by: kbarrett, coleenp, tschatzl

8176054: [BACKOUT][REDO] G1 Needs pre barrier on dereference of weak JNI handles
Reviewed-by: kbarrett, mgerdin

8175085: [REDO] G1 Needs pre barrier on dereference of weak JNI handles
Reviewed-by: kbarrett, dcubed, tschatzl

8175086: [BACKOUT] fix for JDK-8166188
Reviewed-by: kbarrett, jwilhelm, dcubed

8166188: G1 Needs pre barrier on dereference of weak JNI handles
Summary: Add low tag to jweaks and G1 barrier for jweak loads.
Reviewed-by: mgerdin, mdoerr, pliden, dlong, dcubed, coleenp, aph, tschatzl
Contributed-by: kim.barrett@oracle.com, martin.doerr@sap.com, volker.simonis@sap.com