MIPS: Fix big-endian after r21774/r21803.

Fix big-endian ordering of InstanceType and BitField by always loading
the pair as a 16-bit value, even in the API accessor. Clean up some
assertions.

R=danno@chromium.org, mtbrandy@gmail.com

Review URL: https://codereview.chromium.org/334403003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21906 ce2b1a6d-e550-0410-aec6-3dcde31c8c00