[IR] Make Value::getType() work better with invalid IR.

The asmprinter would crash when dumping IR objects that had their
operands dropped.  With this change, we now get this output, which
makes op->dump() style debugging more useful.

%5 = "firrtl.eq"(<<NULL>>, <<NULL>>) : (<<NULL TYPE>>, <<NULL TYPE>>) -> !firrtl.uint<1>

Previously the asmprinter would crash getting the types of the null operands.

Differential Revision: https://reviews.llvm.org/D93869

Author: lattner

mlir/lib/IR/Value.cpp

@@ -32,6 +32,11 @@ Value::Value(Operation *op, unsigned resultNo) {
 
 /// Return the type of this value.
 Type Value::getType() const {
+  // Support a null Value so the asmprinter doesn't crash on invalid IR (e.g.
+  // operations that have dropAllReferences() called on them).
+  if (!*this)
+    return Type();
+
   if (BlockArgument arg = dyn_cast<BlockArgument>())
     return arg.getType();