MIPS: Fix PrepareKeyedOperand on MIPS.

Port r20363 (235f866c)

Original commit message:
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.

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/219923005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20370 ce2b1a6d-e550-0410-aec6-3dcde31c8c00