pan/bi: Introduce TEXC_DUAL psuedoinstruction

pan/bi: Introduce TEXC_DUAL psuedoinstruction

There are two "shapes" of TEXC in the IR:

* Regular texturing. This TEXC writes a single set of staging registers.
* Dual texturing. This TEXC writes two sets of staging registers.

Currently we model both with a 2-destination TEXC, with a null second
destination for the usual case where dual texturing isn't used. This is awkward.
To make the "shapes" of instructions more predictable, make TEXC only write a
single set of staging registers (like the hardware instruction) and split off a
TEXC_DUAL pseudoinstruction for the second case, lowered late.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17794>