freedreno/ir3/cp: properly handle already-folded RELATIV

freedreno/ir3/cp: properly handle already-folded RELATIV

In the `try_swap_mad_two_srcs()` case, valid_flags() gets called both
for the src that we want to try to fold, and for the other src that we
are trying to swap to make that possible.  It can happen in the 2nd case
that a RELATIV src has already been folded.  Since `ssa()` returns non-
null in both the `IR3_REG_SSA` and `IR3_REG_ARRAY` cases (in the later
case, it is the dependent array access that the current instruction
cannot be moved ahead of), we need to explicitly check that the src
reg we are looking at is still an SSA src.

Reported-by: Jonathan Marek <jonathan@marek.ca>
Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5280>