freedreno: a2xx: insert scalar MOV to allow 2 source scalar

If we want to use a scalar instruction with two sources, both sources have
to be in the same register. This covers a common case by inserting a scalar
MOV into a previous instruction with only a vector alu instruction.

A better method would be to have the sources end up in the same register in
the first place, but when one source is a constant this is the only way.

Signed-off-by: Jonathan Marek <jonathan@marek.ca>