Connor Abbott authored Nov 23, 2021 and Marge Bot committed Nov 25, 2021

I was trying to be clever here, skipping ahead to the newly-created
block and processing the remaining instructions after the split in the
same loop. But if the last instruction in a block was lowered, the saved
next instruction would be the head of the block before the split, not
the new block, and we would compare it to the new block so we wouldn't
stop like we were supposed to. Stop being so clever, and just restart
processing with the new block after lowering an instruction.

Because we're wrapping the actual transform in yet another loop, and the
restarting logic is a bit tricky, refactor the actual lowering into a
separate lower_instr function. Otherwise we'd be mixing the two and
indenting the actual logic even more.

Part-of: <mesa/mesa!13928>