[naga] Improve ForbiddenStageOperations error reporting for textureSample in Compute stage

[DavidPeicho]

Description

Using naga to convert a GLSL file using forbidden texture builtin in compute stage leads to a generic:

error: Entry point main at Compute is invalid
= Uses operations forbidden at this stage

Repro steps

Convert this glsl input with naga:

#version 440 core

layout(set = 0, binding = 0) uniform texture2D tex;
layout(set = 0, binding = 1) uniform sampler samp;

layout(local_size_x = 8, local_size_y = 8) in;
void
main()
{
  vec4 data = texture(sampler2D(tex, samp), vec2(0.0, 1.0));
}

Expected behavior

An error message stating that textureSample are forbidden in Compute stages, as written in the WGSL spec: https://www.w3.org/TR/WGSL/#texturesample

Current behavior

error: Entry point main at Compute is invalid
= Uses operations forbidden at this stage

Platform

MacOS, wgpu 23