[spv-in] new block parser (#1294)

* [spv-in] New two pass parser based

* [spv-in] Allow expressions defined in dominant block in different scopes

* Make the patch non breaking

* [spv-in] Allow scope transfers in phi instructions

* [spv-in] Remove unused stuff

* [spv-in] Handle switch merges as breaks

* Remove no longer needed stuff

* Revert some changes to prepare to merge

* Remove dead code

* Don't spill into local if in scope

* [spv-in] Documentation, comments, some renaming for clarity.

* Address comments

Co-authored-by: Jim Blandy <jimb@red-bean.com>