nir: Model AGX-specific multiply-shift-add

nir: Model AGX-specific multiply-shift-add

Models `(a * b) + (c << d)` in general, as implemented in various forms on AGX.
This will be fused with backend NIR opt algebraic rules, both for the literal
pattern as well as to strength reduce certain multiplications, e.g. replacing
a * 5 with `a + (a << 2)` expressed as imadshl_agx(a, 1, a, 2).

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Asahi Lina <lina@asahilina.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22695>