[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