[llvm] [CodeGen] Use MCRegister instead of MCPhysReg in RegisterMaskPair. NFC (PR #123688)

[Matt Arsenault via llvm-commits]

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@@ -895,7 +895,7 @@ void Liveness::computeLiveIns() {
 void Liveness::resetLiveIns() {
   for (auto &B : DFG.getMF()) {
     // Remove all live-ins.
-    std::vector<unsigned> T;
+    std::vector<MCRegister> T;
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arsenm wrote:

This probably should be MCPhysReg 

https://github.com/llvm/llvm-project/pull/123688