When running with compressed references on x86_64, one of the CPU registers holds the heap base pointer to be used for references encoding/decoding. This register is not available for register allocation.

Simple implementations before this release made this register unavailable (and thus unused) even if compressed references were disabled. In this release, the implementation was revised to put this unused register back into the available registers pool. Configurations with large heaps and/or `-XX:-UseCompressedOops` benefit from this improvement.