Small clean up in art::x86_64::ReadBarrierMarkSlowPathX86_64.

Clearly separate art::x86_64::CpuRegister (register object
used by the x86-64 assembler) from art::x86_64::Register
(register number) in
art::x86_64::ReadBarrierMarkSlowPathX86_64::EmitNativeCode.

Test: ART_USE_READ_BARRIER=true make test-art-host
Bug: 12687968
Change-Id: I4300e9b9c16b18119d4e399092aa1c9543518ab5
1 file changed