nir: opt_vectorize: combine different constant sources

We can vectorize instructions with different constant sources by creating
a new load_const and using that.

Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
10 jobs for !1150 with nir-vectorize-const in 12 minutes and 17 seconds (queued for 2 seconds)
merge request