vc4: Actually allow math results to allocate into r4.

vc4: Actually allow math results to allocate into r4.

I switched us to tracking whether the results *could* go to r4, but then
didn't make a separate register class for the class bits that included r4.
Switch the "any" class to actually be "any", and name the "any but r4"
class more appropriately.

total instructions in shared programs: 96798 -> 94680 (-2.19%)
instructions in affected programs:     62736 -> 60618 (-3.38%)