Before:
* IR basic blocks are in arbitrary order
* when doing an IR pass, when a block is encountered, code
must look at all the instructions in the old basic block,
determine what blocks are referenced, and queue up those
old basic blocks first.
* This had a bug (See #667)
Now:
* IR basic blocks are required to be in an order that guarantees
they will be referenced by a branch, before any instructions
within are referenced.
ir pass1 is updated to meet this constraint.
* When doing an IR pass, we iterate over old basic blocks
in the order they appear. Blocks which have not been
referenced are discarded.
* After the pass is complete, we must iterate again to look
for old basic blocks which now point to incomplete new
basic blocks, due to comptime code generation.
* This last part can probably be optimized - most of the time
we don't need to iterate over the basic block again.
closes#667
0ea50b3157