RFR: 8217745: ZGC: Simplify ZLoadBarrierStubC1

[Per Liden]

We can simplify ZLoadBarrierStubC1 a little bit since we know that 
_ref_addr is always a LIR_Address and never a Register. In the 
ZLoadBarrierStubC1 constructor we can just assert that this is always 
true. Later, when emitting code, we can just use the presence of the tmp 
register as indicator that we need to load the effective address.

Bug: https://bugs.openjdk.java.net/browse/JDK-8217745
Webrev: http://cr.openjdk.java.net/~pliden/8217745/webrev.0

/Per