[llvm] [AMDGPU][SILowerSGPRSpills] Correct insertion of IMPLICIT_DEF in cycles (PR #186348)

[Chris Jackson via llvm-commits]

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@@ -35,6 +36,18 @@ using MBBVector = SmallVector<MachineBasicBlock *, 4>;
 
 namespace {
 
+/// Insertion point for IMPLICIT_DEF: iterator may be MBB::end() and can't be
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chrisjbris wrote:

I prefer struct here to give the members names as opposed to std::pairs .first and .second. 

https://github.com/llvm/llvm-project/pull/186348