MIPS: Improve and simplify removal of unreachable code.

Port r17073 (ba68149)

Original commit message:
- Detect unreachable basic blocks of code either following an unconditional deopt or after a provably untaken branch of HBranch or HCompareObjectEqAndBranch instructions.
- Emit dummy uses in unreachable blocks during Hydrogen -> Lithium translation.

BUG=chromium:258519
R=mstarzinger@chromium.org, plind44@gmail.com

Review URL: https://codereview.chromium.org/25708004

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17084 ce2b1a6d-e550-0410-aec6-3dcde31c8c00