Fix handling of enum tags that aren't literals (#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).

2 files changed, 35 insertions, 0 deletions

diff --git a/tests/compute/enum-tag-conversion.slang b/tests/compute/enum-tag-conversion.slang
new file mode 100644
index 000000000..66b4923e5
--- /dev/null
+++ b/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;
+}
\ No newline at end of file
diff --git a/tests/compute/enum-tag-conversion.slang.expected.txt b/tests/compute/enum-tag-conversion.slang.expected.txt
new file mode 100644
index 000000000..a0d427709
--- /dev/null
+++ b/tests/compute/enum-tag-conversion.slang.expected.txt
@@ -0,0 +1,4 @@
+10
+11
+12
+13