HLSL: cast bracket dereference index to int type if not.

HLSL can index arrays using operator[] with non-integer types, and
inserts a conversion to a uint if needed.

Test/hlsl.nonint-index.frag

@@ -0,0 +1,9 @@
+static const float array[3] = { 1, 2, 3 };
+
+float main(float input : IN) : SV_Target0
+{
+    // non-integer indexes get converted to uints:
+    //      uint(input)     3.0          2.0            1.0
+    return array[input] + array[2.0] + array[true] + array[false];
+}
+