[mips] Compact branch policy setting.

This patch adds the commandline option -mcompact-branches={never,optimal,always),
which controls how LLVM generates compact branches for MIPSR6 targets. By default,
the compact branch policy is 'optimal' where LLVM will generate the most
appropriate branch for any situation. The 'never' and 'always' policy will disable
or always generate compact branches wherever possible respectfully.

Reviewers: dsanders, vkalintiris, atanasyan

Differential Revision: http://reviews.llvm.org/D20729

llvm-svn: 271000