[Examples] Add IRTransformations directory to examples.

This patch adds a new IRTransformations directory to llvm/examples/. This is
intended to serve as a new home for example transformations/analysis
code used by various tutorials.

If LLVM_BUILD_EXAMPLES is enabled, the ExamplesIRTransforms library is
linked into the opt binary and the example passes become available.

To start off with, it contains the CFG simplifications used in the IR
part of the 'Getting Started With LLVM: Basics' tutorial at the US LLVM
Developers Meeting 2019.

Reviewers: paquette, jfb, meikeb, lhames, kbarton

Reviewed By: paquette

Differential Revision: https://reviews.llvm.org/D69416