Mark all uses as <undef> when joining a copy.

This way, shrinkToUses() will ignore the instruction that is about to be
deleted, and we avoid leaving invalid live ranges that SplitKit doesn't like.

Fix a misunderstanding in MachineVerifier about <def,undef> operands. The
<undef> flag is valid on def operands where it has the same meaning as <undef>
on a use operand. It only applies to sub-register defines which also read the
full register.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@128642 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/MachineVerifier.cpp

@@ -602,9 +602,7 @@ MachineVerifier::visitMachineOperand(const MachineOperand *MO, unsigned MONum) {
     // Check Live Variables.
     if (MI->isDebugValue()) {
       // Liveness checks are not valid for debug values.
-    } else if (MO->isUndef()) {
-      // An <undef> doesn't refer to any register, so just skip it.
-    } else if (MO->isUse()) {
+    } else if (MO->isUse() && !MO->isUndef()) {
       regsLiveInButUnused.erase(Reg);
 
       bool isKill = false;
@@ -675,8 +673,7 @@ MachineVerifier::visitMachineOperand(const MachineOperand *MO, unsigned MONum) {
             MInfo.vregsLiveIn.insert(std::make_pair(Reg, MI));
         }
       }
-    } else {
-      assert(MO->isDef());
+    } else if (MO->isDef()) {
       // Register defined.
       // TODO: verify that earlyclobber ops are not used.
       if (MO->isDead())