Allow 1D SV_DispatchThreadID in CPU targets (#8612)

The varying param legalization pass didn't deal with this 1D form of
SV_DispatchThreadID for CPU targets:

```slang
void computeMain(int i : SV_DispatchThreadID)
```

Instead, it just overrode the type of `i` with a `uint3`, breaking lots
of code that attempted to use `i` for something, like a `switch`
statement for example.

I ran across this when going through `language-feature` tests for the
LLVM target, which will also use this legalization pass. I'm separately
submitting this now because this also fixes the existing CPU target. The
test I enable in this PR is one that was previously generating broken
code on CPU.

(somewhat related issue: #7468)

1 files changed, 31 insertions, 0 deletions

diff --git a/tests/language-feature/system-value-extraction.slang b/tests/language-feature/system-value-extraction.slang
new file mode 100644
index 000000000..12ca07a96
--- /dev/null
+++ b/tests/language-feature/system-value-extraction.slang
@@ -0,0 +1,31 @@
+//TEST(compute):COMPARE_COMPUTE_EX(filecheck-buffer=CHECK): -dx11 -compute
+//TEST(compute):COMPARE_COMPUTE_EX(filecheck-buffer=CHECK): -dx12 -compute
+//TEST(compute):COMPARE_COMPUTE_EX(filecheck-buffer=CHECK): -cpu -compute
+//TEST(compute):COMPARE_COMPUTE_EX(filecheck-buffer=CHECK): -cuda -compute
+//TEST(compute):COMPARE_COMPUTE_EX(filecheck-buffer=CHECK): -wgsl -compute
+//DISABLE_TEST(compute):COMPARE_COMPUTE_EX(filecheck-buffer=CHECK): -mtl -compute
+//TEST(compute,vulkan):COMPARE_COMPUTE_EX(filecheck-buffer=CHECK): -vk -compute
+
+// Slang allows the type of system value semantics to differ from their
+// "canonical" type - e.g. `uint8_t tid : SV_DispatchThreadID` is technically
+// valid and refers to the first element in the underlying uint3 vector.
+
+// TEST_INPUT: ubuffer(data=[0 0 0 0 0 0 0 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<int> outputBuffer;
+
+[numthreads(2, 4, 1)]
+void computeMain(
+    uint ind : SV_GroupIndex,
+    vector<uint, 2> tid: SV_DispatchThreadID,
+    int localTid : SV_GroupThreadID
+){
+    // CHECK: 0
+    // CHECK-NEXT: 5
+    // CHECK-NEXT: 8
+    // CHECK-NEXT: D
+    // CHECK-NEXT: 10
+    // CHECK-NEXT: 15
+    // CHECK-NEXT: 18
+    // CHECK-NEXT: 1D
+    outputBuffer[ind] = 4 * (tid.x + 2 * tid.y) + localTid;
+}