Translate .Load() to imageLoad() for Vulkan (#967)

* Translate .Load() to imageLoad() for Vulkan

We were already emitting calls to `imageLoad()` and `imageStore` when a `RWTexture*` was used with `operator[]`:

```hlsl
RWTexture2D<float> myTex;
...
float value = myTex[xy]; // becomes an imageLoad
myTex[xy] = value;       // becomes an imageStore
```

However, we were *not* correctly handling the translation of an explicit `.Load()` operation:

```hlsl
float value = myTex.Load(xy);
```

The `.Load()` operation was being translated to a GLSL `texelFetch` as it would be a for a `Texture2D`, and not to an `imageLoad()` as would make sense for a `RWTexture2D` (which becomes a GLSL `image2D`).

This fix is confined to the stdlib, and is mostly a matter of emitting either `texelFetch` or `imageLoad` as the GLSL function name depending on the "access" of the resource type. It is messy code, but straightforward.

One extra detail was that there had been logic to emit a `, 0` argument in the `texelFetch` calls in the non-read-only case, because `texelFetch` usualy requires an explicit mip-level argument and `.Load()` on a `RWTexture*` doesn't recieve an LOD parameter. This is a non-issue now that we are calling `imageLoad()` instead, because `imageLoad` doesn't need/want the extra argument.

* fixup: change test baseline based on recent GLSL output changes

* fixup: review feedback

2 files changed, 42 insertions, 0 deletions

diff --git a/tests/cross-compile/image-load.slang b/tests/cross-compile/image-load.slang
new file mode 100644
index 000000000..7f125bd5a
--- /dev/null
+++ b/tests/cross-compile/image-load.slang
@@ -0,0 +1,19 @@
+// image-load.slang
+
+// This test confirms that `.Load()` on a `RWTexure*`
+// gets properly converted to a call to `imageLoad`
+// and not just `texelFetch` as it would for a `Texture*`.
+
+//TEST:CROSS_COMPILE:-target spirv-assembly -entry main -stage compute
+
+struct Params
+{
+	RWTexture2DArray<float> tex;
+}
+
+ParameterBlock<Params> gParams;
+
+void main(uint3 tid : SV_DispatchThreadID)
+{
+	float f = gParams.tex.Load(int3(tid.xy, tid.z));
+}
diff --git a/tests/cross-compile/image-load.slang.glsl b/tests/cross-compile/image-load.slang.glsl
new file mode 100644
index 000000000..c7233c38c
--- /dev/null
+++ b/tests/cross-compile/image-load.slang.glsl
@@ -0,0 +1,23 @@
+// image-load.slang.glsl
+//TEST_IGNORE_FILE:
+
+#version 450
+
+#extension GL_EXT_samplerless_texture_functions : require
+
+layout(r32f)
+layout(binding = 0)
+uniform image2DArray gParams_tex_0;
+
+layout(local_size_x = 1, local_size_y = 1, local_size_z = 1) in;
+
+void main()
+{
+    float _S1 = imageLoad(
+    	gParams_tex_0,
+    	ivec3(
+    		ivec2(gl_GlobalInvocationID.xy),
+    		int(gl_GlobalInvocationID.z))).x;
+
+    return;
+}