nir/from_ssa: Use nir_instr_free() to free instrs instead of ralloc.

nir/from_ssa: Use nir_instr_free() to free instrs instead of ralloc.

This code was being tricky with passing a mem_ctx instead of the shader,
then freeing the mem_ctx when the pass was done and all the parallel
copies had been removed from the shader.  Use the right type for instr
creation and do a bit of manual list management to prepare the way for
non-ralloc NIR instrs.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11776>