[FIRRTL] Fix signed/unsigned comparison warning

Author: maerhart

lib/Dialect/FIRRTL/FIRRTLInstanceImplementation.cpp

@@ -56,7 +56,7 @@ instance_like_impl::verifyReferencedModule(Operation *instanceOp,
   }
   auto portDirections =
       instanceOp->getAttrOfType<mlir::DenseBoolArrayAttr>("portDirections");
-  if (portDirections.size() != numExpected)
+  if (static_cast<size_t>(portDirections.size()) != numExpected)
     return emitNote(
         instanceOp->emitOpError("the number of port directions should be "
                                 "equal to the number of results"));

lib/Dialect/FIRRTL/FIRRTLOpInterfaces.cpp

@@ -69,7 +69,7 @@ LogicalResult circt::firrtl::verifyModuleLikeOpInterface(FModuleLike module) {
   // TODO: bitwidth is 1 when there are no ports, since APInt previously did not
   // support 0 bit widths.
   auto bitWidth = portDirections.size();
-  if (bitWidth != numPorts)
+  if (static_cast<size_t>(bitWidth) != numPorts)
     return module.emitOpError("requires ") << numPorts << " port directions";
 
   // Verify the port names.