mir-canon: First commit.

mir-canon (MIRCanonicalizerPass) is a pass designed to reorder instructions and
rename operands so that two similar programs will diff more cleanly after being
run through mir-canon than they would otherwise. This project is still a work
in progress and there are ideas still being discussed for improving diff
quality.

M    include/llvm/InitializePasses.h
M    lib/CodeGen/CMakeLists.txt
M    lib/CodeGen/CodeGen.cpp
A    lib/CodeGen/MIRCanonicalizerPass.cpp

llvm-svn: 317285