vc4: Expose r4 to register allocation.

vc4: Expose r4 to register allocation.

We potentially need to be careful that use of a value stored in r4 isn't
copy-propagated (or something) across another r4 write.  That doesn't
appear to happen currently, and this makes the dataflow more obvious.  It
also opens up not unpacking the r4 value, which will be useful for depth
textures.