Fix register trashing in Emit*ByteSeqStringSetChar

This is currently not observable without --allow-natives-syntax because all internal usages are safe, but it deserves to be fixed nonetheless.

BUG=chromium:320922
LOG=N
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/67103003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17873 ce2b1a6d-e550-0410-aec6-3dcde31c8c00