Cache only the last-iteration value of loop PHIs used after loop exit

In split mode (augmented forward + reverse), a loop header PHI chosen to be
cached is stored once per iteration of its loop. When the loop is not rotated,
a running reduction (e.g. `s` in `for (...) s += x[i]*x[i]; return sqrt(s);`)
is the header PHI itself and is used after the loop exits. The reverse pass
then only ever reads the entry of the final iteration, yet the tape holds
trip-count + 1 values (per iteration of any enclosing loop). For a mag() call
inside an O(n) loop this made the tape O(n^2) (2 GB at n = 16000).

A use outside the PHI's loop can only observe the value of the PHI's last
execution before the loop was exited (by dominance the use follows the loop
within the same iteration of every enclosing loop, and the loop cannot be
re-entered without a new iteration of the enclosing loop), independent of
the number of exits. So, if no reverse-pass computation inside the loop needs
the PHI, it suffices to keep one slot per iteration of the enclosing loops
that is overwritten on every iteration; the slot holds the exit value.

computeMinCache (augmented pass only) records such PHIs in
cacheOnlyLastIteration: cached (knownRecomputeHeuristic == false), non-pointer
header PHIs, not needed for loop bounds, all of whose in-loop users are
non-terminator, non-memory, non-pointer instructions that neither use the PHI
directly in their adjoint nor are needed in the reverse pass. For these,
cacheForReverse uses the loop preheader as cache scope (a plain tape value if
the loop is outermost). The decision is stored in AugmentedReturn and reused
by the reverse pass and the split forward-mode derivative, so all tape users
agree on the layout. Anything else keeps the previous behaviour. Combined
mode is unaffected (it already uses fixLCSSA's manual_lcssa phi).

On the reproducer (normalize/mag with only SROA before Enzyme, n = 16000):
peak RSS 1956 MB -> 12 MB, gradient/primal time 5.9x -> 1.2x, identical
gradient.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019RNZGgCdK1fbT2WPQpR7Br