JIT: Port loop compaction to new loop representation (#96995)

Some diffs expected from (at least) three sources:
- Less/different compaction in cases where new loop recognition
  recognizes fewer loops than old recognition
- Less compaction in cases where compacting the first loop led to
  old loop recognition recognizing another loop after the first one.
  This leads to no `BBF_OLD_LOOP_HEADER_QUIRK` set on the second loop's
  header found by new loop finding, meaning we do not compact it. It
  will be compacted as part of a future PR that enables compaction for
  all loops.
- Slight differences in compaction behavior for blocks that aren't part
  of the loop and that couldn't be moved, which would previously abort
  compaction and recognition of the loop. Now we just leave those blocks
  in place.

Author: jakobbotsch

src/coreclr/jit/compiler.h

@@ -6814,7 +6814,12 @@ class Compiler
 
     void optFindLoops();
     void optFindNewLoops();
-    bool optCanonicalizeLoops(FlowGraphNaturalLoops* loops);
+    bool optCanonicalizeLoops();
+    bool optCompactLoops();
+    bool optCompactLoop(FlowGraphNaturalLoop* loop);
+    BasicBlock* optFindLoopCompactionInsertionPoint(FlowGraphNaturalLoop* loop, BasicBlock* top);
+    BasicBlock* optTryAdvanceLoopCompactionInsertionPoint(FlowGraphNaturalLoop* loop, BasicBlock* insertionPoint, BasicBlock* top, BasicBlock* bottom);
+    bool optLoopCompactionFixupFallThrough(BasicBlock* block, BasicBlock* oldNext, BasicBlock* newNext);
     bool optCreatePreheader(FlowGraphNaturalLoop* loop);
     void optSetPreheaderWeight(FlowGraphNaturalLoop* loop, BasicBlock* preheader);