nir: Drop imov/fmov in favor of one mov instruction

nir: Drop imov/fmov in favor of one mov instruction

The difference between imov and fmov has been a constant source of
confusion in NIR for years.  No one really knows why we have two or when
to use one vs. the other.  The real reason is that they do different
things in the presence of source and destination modifiers.  However,
without modifiers (which many back-ends don't have), they are identical.
Now that we've reworked nir_lower_to_source_mods to leave one abs/neg
instruction in place rather than replacing them with imov or fmov
instructions, we don't need two different instructions at all anymore.

Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Vasily Khoruzhick <anarsoul@gmail.com>
Acked-by: Rob Clark <robdclark@chromium.org>