nir: Don't replace the nir_shader when NIR_TEST_CLONE=1

nir: Don't replace the nir_shader when NIR_TEST_CLONE=1

Instead, we add a new helper which stomps one nir_shader and replaces it
with another.  The new helper effectively just changes which pointer
gets used for the base nir_shader.  It should be 99% as good at testing
cloning but without requiring that everything handle having the shader
swapped out from under it constantly.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108957
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Rob Clark <robdclark@chromium.org>