Add SCF support and fence coverage to ElideTimepointsPass. (#22611)

This is a fairly large capability upgrade for existing programs that
used HAL fences to interop with externally allocated or managed buffers
(kvcaches, etc): we can now track the timeline across the HAL ops thanks
to the TimelineAwareOpInterface and fence-like support (we don't use HAL
fences, just say that there are fence-like objects that are not defined
by use-def chains like timepoints are).

SCF support is also added for several cases where we can (today) easily
identify coverage - e.g., scf.if/scf.index_switch are the easy cases,
while we have some initial conservative handling of scf.for/scf.while.
Nesting is supported and we have test coverage for it but there may
still be cases that thwart the analysis and result in extra
joins/awaits.

Part of #16168 PR sequence (5/6).