Finish off SSA

[drcjt]

Started implementing SSA but lots left to do:

• Add ssa number to IR nodes
• Insert Phi Functions
  • Calculate Dominance Frontier
  • Calculate Iterated Dominance Frontier
  • Insert actual phi functions
    • Need to add phi node
• Rename variables

[drcjt]

#353 adds ssa numbers to IR nodes

[drcjt]

Need to write automated tests
Also consider refactoring liveness out of ssabuilder into a separate Liveness class

[drcjt]

Ideas for automated testing of SSA:

• Create new integration test project. For tests assemble il and then use bespoke code to do most of what the MethodCompiler does before running ssa build. Then run the SsaBuilder directly and assert on results e.g. ssa numbers in local variable table
• Add some asserts/checks into the ILCompiler itself to check things, see fgDebugCheckSsa in Ryujit for inspiration
  • There is at most one ssa definition for a given ssa number, and that the definition is in the expected block
  • Sanity check Phi's