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/SelectionDAG/InstrEmitter.cpp

@@ -688,10 +688,13 @@ InstrEmitter::EmitDbgValue(SDDbgValue *SD,
     MIB.addReg(0U);
   }
 
-  if (Offset != 0) // Indirect addressing.
+  // Indirect addressing is indicated by an Imm as the second parameter.
+  if (SD->isIndirect())
     MIB.addImm(Offset);
-  else
+  else {
+    assert(Offset == 0 && "direct value cannot have an offset");
     MIB.addReg(0U, RegState::Debug);
+  }
 
   MIB.addMetadata(MDPtr);