HLSL: Register all entry-point in/out as part of the interface.

This makes the interface be invariant, whether or not individual
variables are used.

Test/baseResults/hlsl.depthLess.frag.out

@@ -13,6 +13,7 @@ using depth_less
 0:3            0.200000
 0:3        Branch: Return
 0:?   Linker Objects
+0:?     '@entryPointOutput' (out float unknown built-in variable)
 
 
 Linked fragment stage:
@@ -32,6 +33,7 @@ using depth_less
 0:3            0.200000
 0:3        Branch: Return
 0:?   Linker Objects
+0:?     '@entryPointOutput' (out float unknown built-in variable)
 
 // Module Version 10000
 // Generated by (magic number): 80001