[mips][mips16] Use isUnconditionalBranch() in AnalyzeBranch() and constant island pass.

Summary:
This stops it misidentifying unconditional branches as conditional branches
which fixes a -verify-machineinstrs error about exiting a function via fall through.

Reviewers: sdardis

Subscribers: dsanders, sdardis, llvm-commits

Differential Revision: http://reviews.llvm.org/D19864

llvm-svn: 268731