nir/clone: Re-use clone_alu for nir_alu_instr_clone

nir/clone: Re-use clone_alu for nir_alu_instr_clone

All it takes are a couple small tweaks to the clone infrastructure to
allow us to use it without any remap table at all.  This reduces code
duplication and the chances for bugs that come with it.  In particular,
the hand-rolled nir_alu_instr_clone didn't preserve no_[un]signed_wrap,
or source/destination modifiers.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5094>