Report more accurately when loop unrolling changes the IR (#80310)

Since unrolling rarely kicks in, reduce JitDump volume by not printing
IR in the JitDump after the unrolling phase unless necessary.

Author: BruceForstall

src/coreclr/jit/optimizer.cpp

@@ -4035,16 +4035,10 @@ PhaseStatus Compiler::optUnrollLoops()
     }
 #endif
 
-#ifdef DEBUG
-    if (verbose)
-    {
-        printf("*************** In optUnrollLoops()\n");
-    }
-#endif
-
     /* Look for loop unrolling candidates */
 
     bool change                 = false;
+    bool anyIRchange            = false;
     bool anyNestedLoopsUnrolled = false;
     INDEBUG(int unrollCount = 0);    // count of loops unrolled
     INDEBUG(int unrollFailures = 0); // count of loops attempted to be unrolled, but failed
@@ -4273,6 +4267,11 @@ PhaseStatus Compiler::optUnrollLoops()
         }
         // clang-format on
 
+        // After this point, assume we've changed the IR. In particular, we call gtSetStmtInfo() which
+        // can modify the IR. We may still fail to unroll if the EH region conditions don't hold, if
+        // the size heuristics don't succeed, or if cloning any individual block fails.
+        anyIRchange = true;
+
         // Heuristic: Estimated cost in code size of the unrolled loop.
 
         {
@@ -4544,6 +4543,8 @@ PhaseStatus Compiler::optUnrollLoops()
 
     if (change)
     {
+        assert(anyIRchange);
+
 #ifdef DEBUG
         if (verbose)
         {
@@ -4588,7 +4589,7 @@ PhaseStatus Compiler::optUnrollLoops()
     fgDebugCheckBBlist(true);
 #endif // DEBUG
 
-    return PhaseStatus::MODIFIED_EVERYTHING;
+    return anyIRchange ? PhaseStatus::MODIFIED_EVERYTHING : PhaseStatus::MODIFIED_NOTHING;
 }
 #ifdef _PREFAST_
 #pragma warning(pop)