A64: Fix int32 use in Lithium string functions

Assert register sizes in StringCharLoadGenerator, and fix char_code comparison
in DoStringCharFromCode.

BUG=
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/172483002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19503 ce2b1a6d-e550-0410-aec6-3dcde31c8c00