Assert that the representation for Load/Store in the X64 MacroAssembler
is never Double. Also add missing test case for the Load/Store macros.

TEST=cctest/test-macro-assembler-x64
R=mvstanton@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=17135

Review URL: https://codereview.chromium.org/25990002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@17151 ce2b1a6d-e550-0410-aec6-3dcde31c8c00