[AMDGPU] `sort` in `AMDGPURewriteAGPRCopyMFMAImpl::eliminateSpillsOfReassignedVGPRs()` needs to have strict weak ordering

[weiweichen]

We ran into an std::sort crash at

llvm-project/llvm/lib/Target/AMDGPU/AMDGPURewriteAGPRCopyMFMA.cpp

Lines 484 to 494 in d0da857

	sort(StackIntervals, [](const LiveInterval *A, const LiveInterval *B) {
	/// Sort heaviest intervals first to prioritize their unspilling
	if (A->weight() > B->weight())
	return true;
	if (A->getSize() > B->getSize())
	return true;
	// Tie breaker by number to avoid need for stable sort
	return A->reg().stackSlotIndex() < B->reg().stackSlotIndex();
	});

with input data

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

The logic for the current comparator of the sort is not strict weak order for this case, because both cmp(a, b) (a.slot < b.slot) and cmp(b, a) (b.size > a.size) are true.

However, std::sort requires strict weak ordering, otherwise it will access out of range / crash with memory corruption:
https://stackoverflow.com/questions/24048022/what-causes-stdsort-to-access-address-out-of-range

https://schneide.blog/2010/11/01/bug-hunting-fun-with-stdsort/#:~:text=Ok%2C%20the%20problem%20is%20that,havoc%20inside%20std::sort.&text=The%20really%20annoying%20thing%20about,give%20you%20lot's%20of%20headaches.

Proposing fix with this change
https://github.com/llvm/llvm-project/pull/162493/files

[llvmbot]

@llvm/issue-subscribers-backend-amdgpu

Author: weiwei chen (weiweichen)

We ran into an `std::sort` crash at https://github.com/llvm/llvm-project/blob/d0da85745dac153705bf1e2c465a8f487068085e/llvm/lib/Target/AMDGPU/AMDGPURewriteAGPRCopyMFMA.cpp#L484-L494 with input data

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

The logic for the current comparator of the sort is not strict weak order for this case, because both cmp(a, b) (a.slot < b.slot) and cmp(b, a) (b.size > a.size) are true.

However, std::sort requires strict weak ordering, otherwise it will access out of range / crash with memory corruption:
https://stackoverflow.com/questions/24048022/what-causes-stdsort-to-access-address-out-of-range
https://schneide.blog/2010/11/01/bug-hunting-fun-with-stdsort/#:~:text=Ok%2C%20the%20problem%20is%20that,havoc%20inside%20std::sort.&amp;text=The%20really%20annoying%20thing%20about,give%20you%20lot's%20of%20headaches.

Proposing to change