pan/bi: Use fused dual source blending

Instead of emitting a pile of moves to fixed registers at codegen time
and hoping everything works out, add a second staging source to the
BLEND instruction in the intermediate representation containing the dual
source colour, and modify register allocation appropriately. This better
models the operation of blending render target #0 with two sources.

Signed-off-by: Alyssa Rosenzweig <>
Part-of: <!13714>
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