Remove dead code in SimplifyCFG

Summary: UsedByBranch is always true according to how BonusInst is defined.

Test Plan:
Passes check-all, and also verified

if (BonusInst && !UsedByBranch) {
  ...
}

is never entered during check-all.

Reviewers: resistor, nadav, jingyue

Reviewed By: jingyue

Subscribers: llvm-commits, eliben, meheff

Differential Revision: http://reviews.llvm.org/D5324