MIPS: Fix LStoreKeyed error when tracing instruction.

Port r14951 (01dbfe2)

Original commit message:
LStoreKeyed can have a NULL value(), when we recognize we are storing a constant
hole value.

BUG=

Review URL: https://codereview.chromium.org/16171019
Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14959 ce2b1a6d-e550-0410-aec6-3dcde31c8c00