The ssa rewrite pass is based on the spirv-opt pass of the same name:
https://github.com/KhronosGroup/SPIRV-Tools/blob/main/source/opt/ssa_rewrite_pass.cpp
This commit brings the pass closer to that reference implementation.
Notably this fixes applying the pass to unstructured graphs which seems to be broken otherwise
Additionally the manual stack iteration of ReadVariable has been replaced with recusion
which is much easier to reason about
Undef handling has also been changed, before each block would define its own Undef instruction,
now the pass will keep track and reuse compatible instructions add insert them to the first block
