[FIRRTL] Add a folder for FIntegerConstantOp.

This isn't used by much yet, but some parts of the MLIR
infrastructure (e.g., Dialect Conversion) assume ConstantLike ops have
a folder. This adds the usual folder, same as StringConstantOp and
others.

include/circt/Dialect/FIRRTL/FIRRTLExpressions.td

@@ -1115,6 +1115,7 @@ def FIntegerConstantOp : FIRRTLOp<"integer", [Pure, ConstantLike]> {
   }];
   let arguments = (ins APSIntAttr:$value);
   let results = (outs FIntegerType:$result);
+  let hasFolder = 1;
   let hasCustomAssemblyFormat = true;
 }
 

lib/Dialect/FIRRTL/FIRRTLFolds.cpp

@@ -410,6 +410,11 @@ OpFoldResult StringConstantOp::fold(FoldAdaptor adaptor) {
   return getValueAttr();
 }
 
+OpFoldResult FIntegerConstantOp::fold(FoldAdaptor adaptor) {
+  assert(adaptor.getOperands().empty() && "constant has no operands");
+  return getValueAttr();
+}
+
 //===----------------------------------------------------------------------===//
 // Binary Operators
 //===----------------------------------------------------------------------===//