DbgIntrinsicInst: Downcast to specialized MDNodes in accessors

Change accessors to downcast to `MDLocalVariable` and `MDExpression`,
now that we have -verify checks in place to confirm that it's safe.

llvm-svn: 232299
diff --git a/llvm/lib/IR/Verifier.cpp b/llvm/lib/IR/Verifier.cpp
index f007106..084bfb8 100644
--- a/llvm/lib/IR/Verifier.cpp
+++ b/llvm/lib/IR/Verifier.cpp
@@ -3032,8 +3032,8 @@
          DII.getRawExpression());
 
   // Don't call visitMDNode(), since that will recurse through operands.
-  visitMDLocalVariable(*cast<MDLocalVariable>(DII.getVariable()));
-  visitMDExpression(*cast<MDExpression>(DII.getExpression()));
+  visitMDLocalVariable(*DII.getVariable());
+  visitMDExpression(*DII.getExpression());
 }
 
 void DebugInfoVerifier::verifyDebugInfo() {