Also check more than one character at a time.
16-bit immediates requires a prefix that changes the length of the instruction.
This causes predecoder mispredictions and subsequent pipeline stalls.
Also removed redundant "atStart" local variable which is equivalent to startIndex == 0.

Review URL: http://codereview.chromium.org/1988009

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4639 ce2b1a6d-e550-0410-aec6-3dcde31c8c00