vc4: Make r4-writes implicitly move to a temp, and allocate temps to r4.

vc4: Make r4-writes implicitly move to a temp, and allocate temps to r4.

Previously, SFU values always moved to a temporary, and TLB color reads
and texture reads always lived in r4.  Instead, we can have these results
just be normal temporaries, and the register allocator can leave the
values in r4 when they don't interfere with anything else using r4.

shader-db results:
total instructions in shared programs: 100809 -> 100040 (-0.76%)
instructions in affected programs:     42383 -> 41614 (-1.81%)