RFR: C1 shenandoah_wb expects obj in a register

[Zhengyu Gu]

Shenandoah_wb expects obj is in a register, so make sure it is.

Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/reg_wb/webrev.00/

Test:

   tier3_gc_shenandoah and java/lang/invoke/VarHandles/*

Thanks,

-Zhengyu