Generate IRType for interfaces, and reference them as `operand[0]` in IRWitnessTable values (#1387)

* Generate IRType for interfaces, and  use them as the type of IRWitnessTable values.

This results the following IR for the included test case:
```
[export("_S3tu010IInterface7Computep1pii")]
let  %1 : _     = key
[export("_ST3tu010IInterface")]
[nameHint("IInterface")]
interface %IInterface   : _(%1);

[export("_S3tu04Impl7Computep1pii")]
[nameHint("Impl.Compute")]
func %Implx5FCompute    : Func(Int, Int)
{
block %2(
                [nameHint("inVal")]
                param %inVal    : Int):
        let  %3 : Int   = mul(%inVal, %inVal)
        return_val(%3)
}
[export("_SW3tu04Impl3tu010IInterface")]
witness_table %4        : %IInterface
{
        witness_table_entry(%1,%Implx5FCompute)

}
```

* Fixes per code review comments.

Moved interface type reference in IRWitnessTable from their type to operand[0].

* Fix typo in comment.

2 files changed, 40 insertions, 0 deletions

diff --git a/tests/compute/dynamic-generics-simple.slang b/tests/compute/dynamic-generics-simple.slang
new file mode 100644
index 000000000..bb009204e
--- /dev/null
+++ b/tests/compute/dynamic-generics-simple.slang
@@ -0,0 +1,36 @@
+//TEST_IGNORE_FILE
+//TEST(compute):COMPARE_COMPUTE:-cpu -xslang -allow-dynamic-code -xslang -dump-ir
+
+// Test basic dynamic dispatch code gen
+
+interface IInterface
+{
+	static int Compute(int inVal);
+};
+
+int GenericCompute<T:IInterface>(int inVal)
+{
+	return T.Compute(inVal);
+}
+
+struct Impl : IInterface
+{
+	static int Compute(int inVal) { return inVal * inVal; }
+};
+
+int test(int inVal)
+{
+	return GenericCompute<Impl>(inVal);
+}
+
+//TEST_INPUT:ubuffer(data=[0 1 2 3], stride=4):out,name=outputBuffer
+RWStructuredBuffer<int> outputBuffer : register(u0);
+
+[numthreads(4, 1, 1)]
+void computeMain(uint3 dispatchThreadID : SV_DispatchThreadID)
+{
+	uint tid = dispatchThreadID.x;
+	int inVal = outputBuffer[tid];
+	int outVal = test(inVal);
+	outputBuffer[tid] = outVal;
+}
\ No newline at end of file
diff --git a/tests/compute/dynamic-generics-simple.slang.expected.txt b/tests/compute/dynamic-generics-simple.slang.expected.txt
new file mode 100644
index 000000000..bf72fb434
--- /dev/null
+++ b/tests/compute/dynamic-generics-simple.slang.expected.txt
@@ -0,0 +1,4 @@
+0
+1
+4
+9
\ No newline at end of file