Add test_b(Operand, immediate) to ia32 assembler, and use it where possible.  Improve comparison to a constant one-character string.  Use CmpInstanceType in more places on ia32.  Add IsObjectJSObjectType and IsInstanceJSObjectType to ia32 macro assembler, using a single branch for a range test.
Review URL: http://codereview.chromium.org/2586001

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4795 ce2b1a6d-e550-0410-aec6-3dcde31c8c00