Simplify a bunch of isVirtualRegister() and isPhysicalRegister() logic.

These functions not longer assert when passed 0, but simply return false instead.

No functional change intended.

llvm-svn: 123155
diff --git a/llvm/lib/CodeGen/TailDuplication.cpp b/llvm/lib/CodeGen/TailDuplication.cpp
index a815b36..ce4b1be 100644
--- a/llvm/lib/CodeGen/TailDuplication.cpp
+++ b/llvm/lib/CodeGen/TailDuplication.cpp
@@ -350,7 +350,7 @@
     if (!MO.isReg())
       continue;
     unsigned Reg = MO.getReg();
-    if (!Reg || TargetRegisterInfo::isPhysicalRegister(Reg))
+    if (!TargetRegisterInfo::isVirtualRegister(Reg))
       continue;
     if (MO.isDef()) {
       const TargetRegisterClass *RC = MRI->getRegClass(Reg);