HLSL: Flesh out the loop grammar and productions.

Test/hlsl.forLoop.frag

@@ -0,0 +1,8 @@
+float4 PixelShaderFunction(float4 input) : COLOR0
+{
+    for (;;) ;
+    for (++input; ; ) ;
+    [unroll] for (; input != input; ) {}
+    for (; input != input; ) { return -input; }
+    for (--input; input != input; input += 2) { return -input; }
+}