r600/sb: fix crash in fold_alu_op3

fold_assoc() called from fold_alu_op3() can lower the number of src to 2,
which then leads to an invalid access to n.src[2]->gvalue().
This didn't seem to have caused much harm in the past, but on Fedora 28
it will crash (presumably because -D_GLIBCXX_ASSERTIONS is used, although
with libstdc++ 4.8.5 this didn't do anything, -D_GLIBCXX_DEBUG was
needed to show the issue).

An alternative fix would be to instead call fold_alu_op2() from within
fold_assoc() when the number of src is reduced and return always TRUE
from fold_assoc() in this case, with the only actual difference being
the return value from fold_alu_op3() then. I'm not sure what the return
value actually should be in this case (or whether it even can make a
difference).

https://bugs.freedesktop.org/show_bug.cgi?id=106928


Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Dave Airlie <airlied@redhat.com>
(cherry picked from commit 817efd89)