Add MIR-level outlining pass

This is a patch for the outliner described in the RFC at:
http://lists.llvm.org/pipermail/llvm-dev/2016-August/104170.html

The outliner is a code-size reduction pass which works by finding
repeated sequences of instructions in a program, and replacing them with
calls to functions. This is useful to people working in low-memory
environments, where sacrificing performance for space is acceptable.

This adds an interprocedural outliner directly before printing assembly.
For reference on how this would work, this patch also includes X86
target hooks and an X86 test.

The outliner is run like so:

clang -mno-red-zone -mllvm -enable-machine-outliner file.c

Patch by Jessica Paquette<jpaquette@apple.com>!

rdar://29166825

Differential Revision: https://reviews.llvm.org/D26872

llvm-svn: 296418