This reapplies r207130 with an additional testcase+and a missing check for

AllocaInst that was missing in one location.
Debug info for optimized code: Support variables that are on the stack and
described by DBG_VALUEs during their lifetime.

Previously, when a variable was at a FrameIndex for any part of its
lifetime, this would shadow all other DBG_VALUEs and only a single
fbreg location would be emitted, which in fact is only valid for a small
range and not the entire lexical scope of the variable. The included
dbg-value-const-byref testcase demonstrates this.

This patch fixes this by
Local
- emitting dbg.value intrinsics for allocas that are passed by reference
- dropping all dbg.declares (they are now fully lowered to dbg.values)
SelectionDAG
- renamed constructors for SDDbgValue for better readability.
- fix UserValue::match() to handle indirect values correctly
- not inserting an MMI table entries for dbg.values that describe allocas.
- lowering dbg.values that describe allocas into *indirect* DBG_VALUEs.
CodeGenPrepare
- leaving dbg.values for an alloca were they are (see comment)
Other
- regenerated/updated instcombine.ll testcase and included source

rdar://problem/16679879
http://reviews.llvm.org/D3374

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207235 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/LiveDebugVariables.cpp

@@ -157,8 +157,8 @@ public:
   UserValue *getNext() const { return next; }
 
   /// match - Does this UserValue match the parameters?
-  bool match(const MDNode *Var, unsigned Offset) const {
-    return Var == variable && Offset == offset;
+  bool match(const MDNode *Var, unsigned Offset, bool indirect) const {
+    return Var == variable && Offset == offset && indirect == IsIndirect;
   }
 
   /// merge - Merge equivalence classes.
@@ -427,7 +427,7 @@ UserValue *LDVImpl::getUserValue(const MDNode *Var, unsigned Offset,
     UserValue *UV = Leader->getLeader();
     Leader = UV;
     for (; UV; UV = UV->getNext())
-      if (UV->match(Var, Offset))
+      if (UV->match(Var, Offset, IsIndirect))
         return UV;
   }