Expand upon existing `ImageSubscript` support (Metal, GLSL, SPIRV) (#4408)

* Add additional `ImageSubscript` features:

1. Added ImageSubscript support for Metal & a test case
    * Merge GLSL/SPIRV/Metal `ImageSubscript` legalization pass
2. Added multisample support to glsl/spirv/metal for when using ImageSubscript
    * Added in this PR since the overhaul of the code merges together GLSL/SPIRV/Metal implementation
3. Fixed minor metal texture `Load`/`Read` bugs
    * [HLSL methods of access do not support subscript accessor for texture cube array](https://learn.microsoft.com/en-us/windows/win32/direct3dhlsl/texturecubearray)
    * removed swizzling of uint/int/float
    * other odd bugs which were causing compile errors

note: Compute tests do not work due to what seems to be the GFX backend (causes crash without error report). The tests are disabled.

* disable LOD with texture 1d

seems that LOD for 1d textures need to be a compile time constant as per an error metal throws

* syntax error in hlsl.meta

* static_assert alone with intrinsic_asm error

provides cleaner errors
Note: `static_assert` seems to be unstable and not be fully respected (still require `intrinsic_asm` to avoid a stdlib compile error)

* change comment to `// lod is not supported for 1D texture

* add `static_assert` in related code gen paths

* address review

* address review

* add asserts as per review comment, NOTE: unclear if these should be release 'asserts' as well

1 files changed, 17 insertions, 2 deletions

diff --git a/source/slang/slang-emit.cpp b/source/slang/slang-emit.cpp
index 0f53f74cd..bb6bab0ab 100644
--- a/source/slang/slang-emit.cpp
+++ b/source/slang/slang-emit.cpp
@@ -50,6 +50,7 @@
 #include "slang-ir-lower-l-value-cast.h"
 #include "slang-ir-lower-reinterpret.h"
 #include "slang-ir-loop-unroll.h"
+#include "slang-ir-legalize-image-subscript.h"
 #include "slang-ir-legalize-vector-types.h"
 #include "slang-ir-metadata.h"
 #include "slang-ir-optix-entry-point-uniforms.h"
@@ -1153,14 +1154,28 @@ Result linkAndOptimizeIR(
     if(isD3DTarget(targetRequest))
         legalizeNonStructParameterToStructForHLSL(irModule);
 
-    // Legalize `ImageSubscript` and constant buffer loads for GLSL.
+    // Legalize `ImageSubscript` loads.
+    switch (target)
+    {
+    case CodeGenTarget::Metal:
+    case CodeGenTarget::GLSL:
+    case CodeGenTarget::SPIRV:
+    case CodeGenTarget::SPIRVAssembly:
+        {
+            legalizeImageSubscript(targetRequest, irModule, sink);
+        } 
+        break;
+    default:
+        break;
+    }
+
+    // Legalize constant buffer loads.
     switch (target)
     {
     case CodeGenTarget::GLSL:
     case CodeGenTarget::SPIRV:
     case CodeGenTarget::SPIRVAssembly:
         {
-            legalizeImageSubscriptForGLSL(irModule);
             legalizeConstantBufferLoadForGLSL(irModule);
             legalizeDispatchMeshPayloadForGLSL(irModule);
         }