Currently, the loop choice condition blocks are created in a way that creates
a confusing flow graph. Restructure them to be simpler, and lay the path for
future work.

The simpler layout (and, hopefully, code and logic to create them) also
lays the groundwork to potentially put the slow path loop in the loop table,
if desired, as it creates a "standard" loop header layout.

While the flow graph has an extra block or two, there are almost no diffs,
as subsequent phases clean up any extras.

There are a few diffs where the new layout enables some downstream optimization
phases that were stymied by the old layout, but mostly in cases where we leave
around dead code slow cloned blocks (a known issue). Also, there are a couple
trivial CSE changes.