pan/bi: Simplify register precolouring in the IR

pan/bi: Simplify register precolouring in the IR

In the current IR, any register may be preloaded by reading it anywhere, and any
register may be precoloured by writing it anywhere. This is convenient for
instruction selection, but requires the register allocator to do considerable
gymnastics to ensure it doesn't clobber precoloured registers. It also breaks
the purity of our SSA representation, which complicates optimization passes
(e.g. copyprop).

Let's trade some instruction selection complexity for simplifying register
allocation by constraining how register precolouring works. Under the new model:

* Registers may only be preloaded at the start of the program.
* Precoloured destinations are handled explicitly by RA.

Internally, a stronger invariant is placed for preloading: registers may only be
preloaded by MOV.i32 instructions at the beginning of the block, and these moves
must be unique. These invariants ensure RA can trivially coalesce the moves.

A bi_preload helper is added as a safe version of bi_register respecting these
invariants, allowing a smooth transition for instruction selection.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16585>