[AMDGPU] Make sort ordering in AMDGPURewriteAGPRCopyMFMAImpl::eliminateSpillsOfReassignedVGPRs() strict weak. (#162493)

- [x] `sort` needs the comparator with strictly weak ordering, however
current logic doesn't meet the
[**Antisymmetry**](https://tanjim131.github.io/2020-05-22-strict-weak-ordering/#:~:text=Almost%20all%20C++%20STL%20containers,the%20person%20with%20greater%20height.)
requirement with

```
sort 0x561ecd3d3db0,0x561eaba91d10  25
  weight 0.000000e+00,0.000000e+00
  size 650370,662754
  slot 732,733
```

Make the comparator logic strict weak order.

Fixes #162490

Author: weiweichen

llvm/lib/Target/AMDGPU/AMDGPURewriteAGPRCopyMFMA.cpp

@@ -482,12 +482,13 @@ void AMDGPURewriteAGPRCopyMFMAImpl::eliminateSpillsOfReassignedVGPRs() const {
   }
 
   sort(StackIntervals, [](const LiveInterval *A, const LiveInterval *B) {
+    // The ordering has to be strictly weak.
     /// Sort heaviest intervals first to prioritize their unspilling
-    if (A->weight() > B->weight())
-      return true;
+    if (A->weight() != B->weight())
+      return A->weight() > B->weight();
 
-    if (A->getSize() > B->getSize())
-      return true;
+    if (A->getSize() != B->getSize())
+      return A->getSize() > B->getSize();
 
     // Tie breaker by number to avoid need for stable sort
     return A->reg().stackSlotIndex() < B->reg().stackSlotIndex();