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.discard.frag.out

@@ -40,6 +40,7 @@ gl_FragCoord origin is upper left
 0:12              0 (const int)
 0:13      Branch: Kill
 0:?   Linker Objects
+0:?     'input' (layout(location=0 ) in 4-component vector of float)
 
 
 Linked fragment stage:
@@ -86,6 +87,7 @@ gl_FragCoord origin is upper left
 0:12              0 (const int)
 0:13      Branch: Kill
 0:?   Linker Objects
+0:?     'input' (layout(location=0 ) in 4-component vector of float)
 
 // Module Version 10000
 // Generated by (magic number): 80001