[mips] Compact branch policy control for MIPSR6

This patch adds the commandline option -mips-compact-branches={never,optimal,always),
which controls how LLVM generates compact branches for MIPS targets. By
default, the compact branch policy is 'optimal' where LLVM will (hopefully)
pick the optimal branch for any situation. The 'never' policy will disable
the generation of compact branches and 'always' will generate compact branches
wherever possible.

Reviewers: dsanders

Differential Review: http://reviews.llvm.org/D20167

llvm-svn: 269753