freedreno/ir3: Cleanup cat6 load instructions

freedreno/ir3: Cleanup cat6 load instructions

There was some src2 vs src3 confusion, but since the syntax is like:

   ldl.f32 rDst, l[rBase+off], ncomp

it makes more sense to call the offset src2 and ncomp src3, than the
way we had it.  This is also easier to deal with for the ir3 assembly
parser.

Also, src_offset was only ever used by the assembly parser, and was
handled incorrectly in emit_cat6(), resulting that cat6 load instrs
would not work properly in (for ex) computerator.  Since we are
cleaning things up, drop src_offset and make the asm parser work in
the same way as the nir->ir3 frontend.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8175>