Fix handling of enum tags that aren't literals (shader-slang#926)

An `enum` case currently lowers to the IR as the value of its tag expression, so if we have:

```hlsl
enum E
{
    X = 99,
}
```

then `X` will lower as IR for the expression `99` which is just a literal.
If instead we have:

```hlsl
enum E
{
    X = 99u,
}
```

then after type checking the expression is `int(99u)`, which will get emitted to the global (module) scope as a cast/conversion instruction, that then gets referenced at the use sites of `E.X`.

The emit logic wasn't set up to handle the case of referencing something directly from the global scope, so this change makes it so that side-effect-free global instructions are treated just like literal constants. This simple handling is fine for now since it will only apply to `enum` tag values (true global constants declared with `static const` have different handling).

Author: Tim Foley

source/slang/emit.cpp

@@ -2393,6 +2393,7 @@ struct EmitVisitor
         case kIROp_GlobalConstant:
         case kIROp_GlobalParam:
         case kIROp_Param:
+        case kIROp_Func:
             return false;
 
         // Always fold these in, because they are trivial
@@ -2513,6 +2514,15 @@ struct EmitVisitor
             }
         }
 
+        // If the instruction is at global scope, then it might represent
+        // a constant (e.g., the value of an enum case).
+        //
+        if(as<IRModuleInst>(inst->getParent()))
+        {
+            if(!inst->mightHaveSideEffects())
+                return true;
+        }
+
         // Having dealt with all of the cases where we *must* fold things
         // above, we can now deal with the more general cases where we
         // *should not* fold things.

tests/compute/enum-tag-conversion.slang

@@ -0,0 +1,31 @@
+// enum-tag-conversion.slang
+
+//TEST(compute):COMPARE_COMPUTE_EX:-slang -compute
+//TEST(compute, vulkan):COMPARE_COMPUTE_EX:-vk -compute
+
+// Confirm that a value of `enum` type can have an initializer
+// that includes basic operations like type conversion.
+
+enum RoseColors
+{
+    Red = 16u,
+}
+
+int test(int val)
+{
+	return val + RoseColors.Red;
+}
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0], stride=4):dxbinding(0),glbinding(0),out
+RWStructuredBuffer<int> outputBuffer;
+
+[numthreads(4, 1, 1)]
+void computeMain(uint3 dispatchThreadID : SV_DispatchThreadID)
+{
+    uint tid = dispatchThreadID.x;
+
+    int val = int(tid);
+    val = test(val);
+
+    outputBuffer[tid] = val;
+}

tests/compute/enum-tag-conversion.slang.expected.txt

@@ -0,0 +1,4 @@
+10
+11
+12
+13