Reapply an improved version of r180816/180817.

Change the informal convention of DBG_VALUE machine instructions so that
we can express a register-indirect address with an offset of 0.
The old convention was that a DBG_VALUE is a register-indirect value if
the offset (operand 1) is nonzero. The new convention is that a DBG_VALUE
is register-indirect if the first operand is a register and the second
operand is an immediate. For plain register values the combination reg,
reg is used. MachineInstrBuilder::BuildMI knows how to build the new
DBG_VALUES.

rdar://problem/13658587

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185966 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/AsmPrinter/DwarfDebug.cpp b/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
index dbac80a..4142480 100644
--- a/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ b/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -1184,7 +1184,8 @@
   assert(MI->isDebugValue() && "Invalid DBG_VALUE machine instruction!");
   return MI->getNumOperands() == 3 &&
          MI->getOperand(0).isReg() && MI->getOperand(0).getReg() &&
-         MI->getOperand(1).isImm() && MI->getOperand(1).getImm() == 0;
+         (MI->getOperand(1).isImm() ||
+          (MI->getOperand(1).isReg() && MI->getOperand(1).getReg() == 0U));
 }
 
 // Get .debug_loc entry for the instruction range starting at MI.
@@ -1195,12 +1196,11 @@
   const MDNode *Var =  MI->getOperand(MI->getNumOperands() - 1).getMetadata();
 
   assert(MI->getNumOperands() == 3);
-  if (MI->getOperand(0).isReg() && MI->getOperand(1).isImm()) {
+  if (MI->getOperand(0).isReg()) {
     MachineLocation MLoc;
-    // TODO: Currently an offset of 0 in a DBG_VALUE means
-    // we need to generate a direct register value.
-    // There is no way to specify an indirect value with offset 0.
-    if (MI->getOperand(1).getImm() == 0)
+    // If the second operand is an immediate, this is a
+    // register-indirect address.
+    if (!MI->getOperand(1).isImm())
       MLoc.set(MI->getOperand(0).getReg());
     else
       MLoc.set(MI->getOperand(0).getReg(), MI->getOperand(1).getImm());