[LoopUnroll] Consider convergence control tokens when unrolling (#91715)

- There is no restriction on a loop with controlled convergent
operations when
  the relevant tokens are defined and used within the loop.

- When a token defined outside a loop is used inside (also called a loop
convergence heart), unrolling is allowed only in the absence of
remainder or
  runtime checks.

- When a token defined inside a loop is used outside, such a loop is
said to be
"extended". This loop can only be unrolled by also duplicating the
extended part
  lying outside the loop. Such unrolling is disabled for now.

- Clean up loop hearts: When unrolling a loop with a heart, duplicating
the
heart will introduce multiple static uses of a convergence control token
in a
cycle that does not contain its definition. This violates the static
rules for
tokens, and needs to be cleaned up into a single occurrence of the
intrinsic.

- Spell out the initializer for UnrollLoopOptions to improve
readability.


Original implementation [D85605] by Nicolai Haehnle
<nicolai.haehnle@amd.com>.