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.precise.frag.out

@@ -26,6 +26,7 @@ gl_FragCoord origin is upper left
 0:12        Branch: Return
 0:?   Linker Objects
 0:?     'precisefloat' (noContraction global float)
+0:?     '@entryPointOutput' (out structure{noContraction temp 4-component vector of float color})
 
 
 Linked fragment stage:
@@ -58,6 +59,7 @@ gl_FragCoord origin is upper left
 0:12        Branch: Return
 0:?   Linker Objects
 0:?     'precisefloat' (noContraction global float)
+0:?     '@entryPointOutput' (out structure{noContraction temp 4-component vector of float color})
 
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