[llvm] [AMDGPU][CodeGen] Utilized MBB's liveIn check in SILowerSGPRSpills (PR #129848)

[Matt Arsenault via llvm-commits]

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@@ -177,6 +179,10 @@ class MachineBasicBlock
   using LiveInVector = std::vector<RegisterMaskPair>;
   LiveInVector LiveIns;
 
+  /// Keeps track of live register units for those physical registers which
+  /// are livein of the basicblock.
+  BitVector LiveInRegUnits;
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arsenm wrote:

Blocks should not track both LiveInRegUnits and LiveIns. We should change the representation here, but you should not do that as a part of a patch to fix SILowerSGPRSpills. You do not need to change the internal representation to fix the downstream use issue 

https://github.com/llvm/llvm-project/pull/129848