pan/bi: Fix unpack_32_2x16 definition

This got messed up when scalarizing the IR. Fix the definition of the opcode to
return (instead of break, asserting out) and to respect the swizzle (instead of
failing validation). Noticed when bringing up OpenCL on Valhall.

Fixes: 5febeae5 ("pan/bi: Emit collect and split")
Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <!17222>
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