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.constructexpr.frag.out

@@ -40,6 +40,7 @@ gl_FragCoord origin is upper left
 0:16          'ps_output' (temp structure{temp 4-component vector of float color})
 0:16        Branch: Return
 0:?   Linker Objects
+0:?     '@entryPointOutput' (out structure{temp 4-component vector of float color})
 
 
 Linked fragment stage:
@@ -86,6 +87,7 @@ gl_FragCoord origin is upper left
 0:16          'ps_output' (temp structure{temp 4-component vector of float color})
 0:16        Branch: Return
 0:?   Linker Objects
+0:?     '@entryPointOutput' (out structure{temp 4-component vector of float color})
 
 // Module Version 10000
 // Generated by (magic number): 80001