Fix PrepareKeyedOperand on arm.

When additional_offset is specified, the 'key' operand can be negative
and still pass the bounds check. Therefore, when converting key from
Smi, arithmetic and not logical shift must be used.

R=verwaest@chromium.org
BUG=358057
LOG=Y

Review URL: https://codereview.chromium.org/219473002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20363 ce2b1a6d-e550-0410-aec6-3dcde31c8c00