Check quantized-storage-type to be a signless integer

[sdasgup3]

Current StablehLO quantized signed int cs uses mlir::quant::QuantizedType::isSIgned to decide on the signed-ness of the storage type.

StableHLO, as bootstrapped from MHLO, inherits signless integers (added in MHLO for some legacy reasons) to be treated as signed integer. This is not captured by the check mlir::quant::QuantizedType::isSigned because of the following reason.

Based on this, mlir::quant::QuantizedType::isSIgned() is true if the associated bit in flag is 1. Here are a few ways to set that flag as true.

 auto signed_flag_bit = storage_type.cast<mlir::IntegerType>().isSignless();
... mlir::quant::UniformQuantizedPerAxisType::get(
     is_signed, storage_type, expressed_type, scales, zero_points,
     quantization_dimension, storage_min, storage_max);

or

 auto signed_flag_bit = storage_type.cast<mlir::IntegerType>().isSigned();
... mlir::quant::UniformQuantizedPerAxisType::get(
     is_signed, storage_type, expressed_type, scales, zero_points,
     quantization_dimension, storage_min, storage_max);

In other words, It is on the producers of mlir::quant::QuantizedType to set that bit based on their interpretation of signedness, which in case of StableHLO is signedless. That means the a true value of mlir::quant::QuantizedType::isSIgned() is not enough to ensure that the desired signed-ness of storage type.

Suggested change

Replace

CPred<"$_self.cast<mlir::quant::UniformQuantizedType>()" #
                 ".isSigned()">]>,

with

           CPred<"$_self.cast<mlir::quant::UniformQuantizedType>()" #
                 ".getStorageType().cast<mlir::IntegerType>().isSignless()">]>,

IMO, we can skip the check mlir::quant::UniformQuantizedType::isSigned() altogether as is it not sufficient nor necessary.

Context: #1812 (comment)