vc4: Use register allocator CLASS_BIT_R0_R3 to clean up CLASS_B.
vc4: Use register allocator CLASS_BIT_R0_R3 to clean up CLASS_B.

We have had no reason to separate ability to store in an accumulator from
ability to store in B, but with FS threading, we need to be able to force
values to be stored only in the physical regfiles.

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler