[llvm] [CodeGen] Rename RegisterMaskPair to RegUnitMaskPair. NFC (PR #123799)

[Matt Arsenault via llvm-commits]

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@@ -35,11 +35,11 @@ class MachineInstr;
 class MachineRegisterInfo;
 class RegisterClassInfo;
 
-struct RegisterMaskPair {
+struct RegUnitMaskPair {
   Register RegUnit; ///< Virtual register or register unit.
   LaneBitmask LaneMask;
 
-  RegisterMaskPair(Register RegUnit, LaneBitmask LaneMask)
+  RegUnitMaskPair(Register RegUnit, LaneBitmask LaneMask)
       : RegUnit(RegUnit), LaneMask(LaneMask) {}
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arsenm wrote:

This is really something like `std::variant<unsigned RegUnit, std::pair<Register, LaneBitmask>>`.

Maybe LiveInReg? That's all this is used for I believe 

https://github.com/llvm/llvm-project/pull/123799