verify the structure of basic blocks

[mnikander]

• created a separate preprocessing pass as part of the static analysis
• enforce static single assignment (each variable only defined once in the source code)
• enforce labels at the start of every block
• enforce phi nodes at the start of the block, no other instructions before the phi nodes
• enforce terminator instructions only at the end of a block
• enforce that all variables which are used somewhere, are actually defined somewhere
• design decisions documented for each of these properties
• revise the design decision about not validating the structure of the input code (there are a lot of properties which must be upheld, so it's worth checking them to avoid weird bugs)
• document the decision to use typed javascript arrays for the input: this allows the type-checker to check them, so a separate pass to check the validity of the instructions is not required

[mnikander]

There is no need to check for a valid structure of the instructions though, since those are javascript arrays which are checked by the typechecker when compiling the code