MIPS: Lithium codegen should not pass around block_ids.

Port r15235 (95a844b)

Original commit message:
Rather encapsulate the basic block to assembly label mapping in the LInstruction.

BUG=

Review URL: https://codereview.chromium.org/17504002
Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15257 ce2b1a6d-e550-0410-aec6-3dcde31c8c00