MIPS: Remove extranous holder load in some prototype-chain checking cases

Port r14136 (d091bc0e)

BUG=

Review URL: https://codereview.chromium.org/13636002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14142 ce2b1a6d-e550-0410-aec6-3dcde31c8c00