RFR: 8235729: Shenandoah: Remove useless casting to non-constant

[Roman Kennke]

We have a couple of code-paths in Shenandoah's C2 barrier code that cast 
values to non-constant. This appears to be a left-over from pre-LRB 
barrier scheme.

This patch removes those casts as well as the relevant methods in 
type.hpp/cpp. Shenandoah has been the only user of this.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8235729
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8235729/webrev.00/

Testing: hotspot_gc_shenandoah, ctw-tests with default and traversal mode

Can I please get a review of the change?

Thanks,
Roman