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.

also fixes ADD and SUB to use U modifier

Along with implementations of common instructions along the way