nv50/ir: fix combineLd/St to update existing records as necessary

nv50/ir: fix combineLd/St to update existing records as necessary

Previously the logic would decide that the record is kept, which
translates into keep = false in the caller, which meant that these
passes did not run.

While it's right that keep = false which means that a new record does
not need to be added, we do still have to perform the usual list
maintenance. It's easiest to do this pre-merge rather than post.

The lowering that clip/cull distance passes produce triggers this bug in
TCS (since reading outputs is done differently in other stages), but it
should be possible to achieve it with the right sequence of regular
reads/writes.

Fixes: KHR-GL45.cull_distance.functional
Fixes: generated_tests/spec/arb_tessellation_shader/execution/tes-input/tes-input-gl_ClipDistance.shader_test
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Cc: mesa-stable@lists.freedesktop.org