Fix MLIR bytecode reading of i0 IntegerAttr

[mehdi_amini]

Summary

The move of the bytecode serialization to be tablegen driven in
https://reviews.llvm.org/D144820 added a new condition in the reading
path that forbid 0-sized integer, even though we still produce them.

Fix #62920

mlir/lib/IR/BuiltinDialectBytecode.cpp

static unsigned getIntegerBitWidth(DialectBytecodeReader &reader, Type type) {
   reader.emitError()
       << "expected integer or index type for IntegerAttr, but got: " << type;
   return 0;
 }
 
 static LogicalResult readAPIntWithKnownWidth(DialectBytecodeReader &reader,
                                              Type type, FailureOr<APInt> &val) {
   unsigned bitWidth = getIntegerBitWidth(reader, type);
-  if (bitWidth == 0)
-    return failure();
   val = reader.readAPIntWithKnownWidth(bitWidth);
   return val;
 }
 
 static LogicalResult
 readAPFloatWithKnownSemantics(DialectBytecodeReader &reader, Type type,
                               FailureOr<APFloat> &val) {
   auto ftype = dyn_cast<FloatType>(type);

mlir/test/Bytecode/empty_attr.mlir

+// RUN: mlir-opt -emit-bytecode %s | mlir-opt | FileCheck %s
+
+// CHECK: module
+// CHECK: foo.asdf = 0 : i0
+module attributes { foo.asdf = 0 : i0 } { }