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nir/opt_16b_tex_image: Sign extension should matter for texel buffer txf

Danylo Piliaiev requested to merge Danil/mesa:nir/fix/sign-extension-matters-for-texel-buffer into main

What does this MR do and why?

Texel buffer could be arbitrary large, so the assumption being made in the following comment is wrong:

 "Zero-extension (u16) and sign-extension (i16) have
  the same behavior here - txf returns 0 if bit 15 is set
  because it's out of bounds and the higher bits don't matter."

Sign extension should matter for GLSL_SAMPLER_DIM_BUF.

This fixes the case of doing texelFetch with u16 offset:

  uniform itextureBuffer s1;
  uint16_t offset = some_ssbo.offset;
  value = texelFetch(s1, offset).x;

If the offset is higher than s16 optimization incorrectly left it as 16b.

In spirv the above glsl is translated into:

  %22 = OpLoad %ushort %21
  %23 = OpUConvert %uint %22
  %24 = OpBitcast %int %23
  %26 = OpImageFetch %v4int %16 %24

Cc: mesa-stable

Example reproducer for Turnip:

#!amber

SHADER compute compute_shader GLSL
#version 450

#extension GL_EXT_shader_explicit_arithmetic_types_int16 : require
#extension GL_EXT_shader_16bit_storage : require

layout(set = 0, binding = 0) buffer block {
    int value;
    u16vec2 offset;
};

layout(set = 0, binding = 1) uniform itextureBuffer s1;

void
main()
{
    value = texelFetch(s1, offset.x).x;
}
END

BUFFER buf_in DATA_TYPE R16G16_SINT SIZE 65536 FILL 7

BUFFER buf_out DATA_TYPE uint16 DATA 0 0 32768 32769 END

PIPELINE compute pipeline
  ATTACH compute_shader

  BIND BUFFER buf_out AS storage DESCRIPTOR_SET 0 BINDING 0
  BIND BUFFER buf_in AS uniform_texel_buffer DESCRIPTOR_SET 0 BINDING 1
END

RUN pipeline 1 1 1

EXPECT buf_out IDX 0 EQ 7

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