[LivePhysRegs] Use .asMCReg() rather than cast (NFC) (llvm#155084)

Should fix MSVC buildbots:

https://lab.llvm.org/buildbot/#/builders/197/builds/8321
https://lab.llvm.org/buildbot/#/builders/211/builds/1549

Author: MacDue

llvm/lib/CodeGen/LivePhysRegs.cpp

@@ -366,7 +366,7 @@ bool llvm::isPhysRegUsedAfter(Register Reg, MachineBasicBlock::iterator MBI) {
   // If we hit the end of the block, check whether Reg is live into a
   //  successor.
   for (const auto &LO : MBB->liveouts())
-    if (LO.PhysReg == MCRegister(Reg) && LO.LaneMask.any())
+    if (LO.PhysReg == Reg.asMCReg() && LO.LaneMask.any())
       return true;
 
   return false;