RFR (S) 8229419: Shenandoah: Cleanup LRB strength selector code

[Roman Kennke]

>> RFE:
>>  https://bugs.openjdk.java.net/browse/JDK-8229419
>>  https://cr.openjdk.java.net/~shade/8229419/webrev.01/
> 
> https://cr.openjdk.java.net/~shade/8229419/webrev.02/
> 
> Last minute cleanup:
>  - invert Op_CmpP condition from && to ||, makes simpler statement;
>  - return strength=NONE from "Do not need barriers" block;

This is wrong, IMO. For several uses of a barrier we want the strongest
one. Returning NONE means 'there is no stronger use than NONE'. If the
next sibling would be STRONG, then we'd elide the barrier even though we
must not. Please leave the previous "// NONE, i.e. leave current
strength as is".

Roman