[FIRRTL] Reconsider use of inferReturnType in FIRRTLExprs

[mikeurbach]

We require FIRRTLExprs to define inferReturnType, which is used, for example, here:

circt/lib/Dialect/FIRRTL/Import/FIRParser.cpp

Line 2285 in a3e993c

auto resultTy = CLASS::inferReturnType(operands, attrs, {}); \

There is some ODS machinery to allow subclasses to customize this using inferTypeDecl and inferType:

circt/include/circt/Dialect/FIRRTL/FIRRTLExpressions.td

Lines 548 to 566 in a3e993c

	// Give concrete operations a chance to set a type inference callback. If left
	// empty, a declaration for `inferBinaryReturnType` will be emitted that the
	// operation is expected to implement.
	code inferType = "";
	let inferTypeDecl = !if(!empty(inferType), [{
	/// Infer the return type of this binary operation.
	static FIRRTLType inferBinaryReturnType(FIRRTLType lhs, FIRRTLType rhs,
	std::optional<Location> loc);
	}], "") # !subst("$_infer", !if(!empty(inferType), "inferBinaryReturnType",
	inferType), [{
	/// Infer the return type of this operation.
	static FIRRTLType inferReturnType(ValueRange operands,
	ArrayRef<NamedAttribute> attrs,
	std::optional<Location> loc) {
	return $_infer(firrtl::type_cast<FIRRTLType>(operands[0].getType()),
	firrtl::type_cast<FIRRTLType>(operands[1].getType()),
	loc);
	}
	}]);

@uenoku pointed out on #6701 that we can probably just use methods from InferTypeOpInterface. All of this machinery seems to predate that existing, but we might be able to simplify it now.