MIPS: Fix the hole loading optimization.

Port r14829 (8fba0f34)

Original commit message:
Fix the hole loading optimization.
- Holes are only ever loaded as double or tagged.
- Change to tagged has to deoptimize on undefined (no implicit   conversions from double the hole NaN -> tagged undefined).

BUG=

Review URL: https://codereview.chromium.org/16116002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14831 ce2b1a6d-e550-0410-aec6-3dcde31c8c00