HLSL: Fix #1815: Don't constant fold spec-constant array indexing.

Test/hlsl.specConstant.frag

@@ -0,0 +1,15 @@
+[[vk::constant_id(0)]] const uint index = 2;
+
+static const uint array[] = { 10, 20, 30, 40 };
+
+float4 main( ) : SV_TARGET
+{
+    float4 r;
+    for(uint i = 0; i < array[index]; i++)
+        r = i;
+
+    r += index + index;
+    r += 2 * index;
+
+    return r;
+}