TosaToLinalg: Support unsigned tosa.max_pool2d (FXML-4556)

[mgehre-amd]

This PR allows to lower unsigned tosa.max_pool2d to linalg.

// CHECK-LABEL: @max_pool_ui8
func.func @max_pool_ui8(%arg0: tensor<1x6x34x62xui8>) -> tensor<1x4x32x62xui8> {
  // CHECK: builtin.unrealized_conversion_cast {{.*}} : tensor<1x6x34x62xui8> to tensor<1x6x34x62xi8>
  // CHECK: arith.constant 0
  // CHECK: linalg.pooling_nhwc_max_unsigned {{.*}} : (tensor<1x4x32x62xi8>) -> tensor<1x4x32x62xi8>
  // CHECK: builtin.unrealized_conversion_cast {{.*}} : tensor<1x4x32x62xi8> to tensor<1x4x32x62xui8>
  %0 = tosa.max_pool2d %arg0 {pad = array<i64: 0, 0, 0, 0>, kernel = array<i64: 3, 3>, stride = array<i64: 1, 1>} : (tensor<1x6x34x62xui8>) -> tensor<1x4x32x62xui8>
  return %0 : tensor<1x4x32x62xui8>
}

It does this by

1. Moving the type converter from mlir/lib/Conversion/TosaToLinalg/TosaToLinalgPass.cpp into populateTosaToLinalgTypeConversion
2. Adding materializations to the type converter using UnrealizedConversionCastOp
3. Converting the MaxPool2dConverter from OpRewriterPattern to OpConversion Pattern
4. Adjusting the padding value to the the minimum unsigned value when the max_pool is unsigned
5. Lowering to linalg.pooling_nhwc_max_unsigned (which uses arith.maxui) when the max_pool is unsigned

[josel-amd]

LGTM

[cferry-AMD]

LGTM. Let's see what upstream folks think for the interface; I'd prefer the approach proposed here but there may be a use case that needs it changed.