DebugInfo: Assert dbg.declare/value insts are valid

Remove early returns for when `getVariable()` is null, and just assert
that it never happens.  The Verifier already confirms that there's a
valid variable on these intrinsics, so we should assume the debug info
isn't broken.  I also updated a check for a `!dbg` attachment, which the
Verifier similarly guarantees.

llvm-svn: 235400
diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index 1c9760e..5672a36 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -1000,8 +1000,7 @@
                                            StoreInst *SI, DIBuilder &Builder) {
   DIVariable DIVar = DDI->getVariable();
   DIExpression DIExpr = DDI->getExpression();
-  if (!DIVar)
-    return false;
+  assert(DIVar && "Missing variable");
 
   if (LdStHasDebugValue(DIVar, SI))
     return true;
@@ -1028,8 +1027,7 @@
                                            LoadInst *LI, DIBuilder &Builder) {
   DIVariable DIVar = DDI->getVariable();
   DIExpression DIExpr = DDI->getExpression();
-  if (!DIVar)
-    return false;
+  assert(DIVar && "Missing variable");
 
   if (LdStHasDebugValue(DIVar, LI))
     return true;
@@ -1107,8 +1105,7 @@
   DebugLoc Loc = DDI->getDebugLoc();
   DIVariable DIVar = DDI->getVariable();
   DIExpression DIExpr = DDI->getExpression();
-  if (!DIVar)
-    return false;
+  assert(DIVar && "Missing variable");
 
   if (Deref) {
     // Create a copy of the original DIDescriptor for user variable, prepending