Silence unused variable warning

This commit regresses VertexA shaders, their transformation pass has to
be adapted to the new control flow.

StorageAtomicExchangeU64 is failing test seemingly due to failure storing 64-bit
result into the register

Use a struct constructor to serialize register allocation arguments to
ensure registers are allocated in the same order regardless of the
compiler used.

The A and B functions can be called in any order when passed as
arguments to "foo":

  foo(A(), B())

But the order is guaranteed for curly-braced constructor calls in
classes:

  Foo{A(), B()}

Use this to get consistent behavior.