freedreno/a3xx/compiler: better const handling

Seems like most/all instructions have some restrictions about const src
registers.  In seems like the 2 src (cat2) instructions can take at most
one const, and the 3 src (cat3) instructions can take at most one const
in the first 2 arguments.  And so on.  Handle this properly now.

Signed-off-by: Rob Clark <robclark@freedesktop.org>