MIPS: Fixed a bug in CopyBytes() and new test cases for MIPS macro assembler.

Port r16873 (c2880bc)

BUG=
TEST=cctest/test-macro-assembler-mips
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/24266007

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16904 ce2b1a6d-e550-0410-aec6-3dcde31c8c00