[llvm] Make default size MBB.size() (PR #83526)

[Matt Arsenault via llvm-commits]

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@@ -1641,8 +1642,7 @@ MachineBasicBlock::computeRegisterLiveness(const TargetRegisterInfo *TRI,
     return LQR_Dead;
   }
 
-
-  N = Neighborhood;
+  N = this->size();
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arsenm wrote:

Searching the entire block entirely defeats the purpose of having the threshold 

https://github.com/llvm/llvm-project/pull/83526