[NFC] cache repeated tree walks to avoid O(N^2) in optimizeTerminatingTails in CodeFolding

[Changqing-JING]

Cache the result of getBranchTargets(getFunction()->body) in optimizeTerminatingTails so that recursive calls share the same computed set rather than each re-walking the entire function body. This avoids O(N²) behavior where N is the size of the function body, since the recursive calls previously each performed an O(N) tree walk. The cached targets are computed lazily on first need and passed through to the canMove overload that accepts pre-computed branch targets.

Benmark data

For the test case in #7319 (comment)
Main head:

time ./build/bin/wasm-opt --code-folding --enable-bulk-memory --enable-multivalue --enable-reference-types --enable-gc --enable-tail-call --enable-exception-handling -o /dev/null ./test3.wasm

real    5m45.996s
user    6m6.267s
sys     0m3.798s

This PR:

time ./build/bin/wasm-opt --code-folding --enable-bulk-memory --enable-multivalue --enable-reference-types --enable-gc --enable-tail-call --enable-exception-handling -o /dev/null ./test3.wasm

real    2m2.380s
user    2m25.700s
sys     0m2.449s

Benchmark regression test

Test case: https://jetbrains.github.io/kotlinconf-app/73cbe24d7cf5a54d37ad.wasm
On main

Performance counter stats for 'build/bin/wasm-opt 73cbe24d7cf5a54d37ad.wasm -all --code-folding -o /dev/null' (10 runs):

        4837936912      task-clock                       #    1.445 CPUs utilized               ( +-  0.51% )
               114      context-switches                 #   23.564 /sec                        ( +-  7.58% )
                 7      cpu-migrations                   #    1.447 /sec                        ( +- 16.88% )
             46271      page-faults                      #    9.564 K/sec                       ( +-  0.00% )
       13431328103      instructions                     #    1.21  insn per cycle              ( +-  0.01% )
       11125222873      cycles                           #    2.300 GHz                         ( +-  0.51% )
          64641504      branch-misses                                                           ( +-  1.26% )

            3.3484 +- 0.0221 seconds time elapsed  ( +-  0.66% )

On current PR

 Performance counter stats for 'build/bin/wasm-opt 73cbe24d7cf5a54d37ad.wasm -all --code-folding -o /dev/null' (10 runs):

        4802304211      task-clock                       #    1.437 CPUs utilized               ( +-  0.47% )
               125      context-switches                 #   26.029 /sec                        ( +-  6.50% )
                 8      cpu-migrations                   #    1.666 /sec                        ( +- 14.20% )
             46272      page-faults                      #    9.635 K/sec                       ( +-  0.00% )
       13391520427      instructions                     #    1.21  insn per cycle              ( +-  0.01% )
       11043221889      cycles                           #    2.300 GHz                         ( +-  0.47% )
          59021679      branch-misses                                                           ( +-  1.24% )

            3.3427 +- 0.0207 seconds time elapsed  ( +-  0.62% )

[Changqing-JING]

@kripken
Could you help to review this PR?

[tlively]

LGMT with some extra comments.