pan/bi: Fix assert when writing vertex outputs

Varying stores require us to generate a LEA_ATTR_IMM instruction, which
is a load.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Fixes: 0aa08ae2 ("nir: Split NIR_INTRINSIC_TYPE into separate src/dest indices")
Part-of: <!6985>
37 jobs for !6985 with hotfix in 9 minutes and 24 seconds (queued for 4 seconds)
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