[MBP] Don't outline short optional branches

With the option -outline-optional-branches, LLVM will place optional
branches out of line (more details on r231230).

With this patch, this is not done for short optional branches. A short
optional branch is a branch containing a single block with an
instruction count below a certain threshold (defaulting to 3). Still
everything is guarded under -outline-optional-branches).

Outlining a short branch can't significantly improve code locality. It
can however decrease performance because of the additional jmp and in
cases where the optional branch is hot. This fixes a compile time
regression I have observed in a benchmark.

Review: http://reviews.llvm.org/D8108
llvm-svn: 232802