freedreno/ir3/cp: support to swap mad src's

freedreno/ir3/cp: support to swap mad src's

For a normal MAD (ie. not MADSH), if first source is gpr and second
source is const, we can swap the first two sources to avoid needing a
mov instruction.

This gives back the biggest advantage TGSI f/e had over NIR f/e for
common shaders, since TGSI f/e had this logic in the f/e.  Note that
doing this in copy-prop step has the advantage that it will also work
for cases like:

   MOV TEMP[b], CONST[x]
   MAD TEMP[d], TEMP[a], TEMP[b], TEMP[c]

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