JIT: fix jit stress failures in fgSetBlockOrder (#80975)

Defer asserting that bbNums are ordered until we know if we're optimizing
(and hence have computed dominators).

Fixes #80974.

src/coreclr/jit/optimizer.cpp

@@ -4692,8 +4692,6 @@ bool Compiler::optReachWithoutCall(BasicBlock* topBB, BasicBlock* botBB)
     // When we can determine this, then we can set BBF_GC_SAFE_POINT for
     // those helpers too.
 
-    noway_assert(topBB->bbNum <= botBB->bbNum);
-
     // We can always check topBB and botBB for any gc safe points and early out
 
     if ((topBB->bbFlags | botBB->bbFlags) & BBF_GC_SAFE_POINT)
@@ -4709,6 +4707,8 @@ bool Compiler::optReachWithoutCall(BasicBlock* topBB, BasicBlock* botBB)
         return true;
     }
 
+    noway_assert(topBB->bbNum <= botBB->bbNum);
+
     BasicBlock* curBB = topBB;
     for (;;)
     {