Spits out a graphviz .dot file describing the control flow of the shader.

This is very similar to the output of the spirv-cfg tool in SPIRV-Tools. This is just much easier to use, and allows us to add more bespoke information to the graph in the future.

Bug: b/140287657
Change-Id: Id90b065a3599b941b192a5e8105dffac9ba8f566
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/32952
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>