There was lots of duplicate code here.

Add SpirvRoutine::setImmutableInputBuiltins() to de-duplicate the setting of compile-time constant values.
Add SpirvRoutine::setInputBuiltin() to unify the method for setting input builtins.

Change-Id: If3c2632b4fdd74f050d2dc1494349ca359ea92fb
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/35108
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Tested-by: Ben Clayton <bclayton@google.com>