Additional Phi node semantics details for ssair docs (#52587)
Following a discussion on slack, I thought I would propose to add this
to the docs. I'm very open to changing the form of the explanation, I
just wanted to get the point across that "all phi nodes at the start of
a basic block run simultaneously".
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Co-authored-by: Keno Fischer <keno@alumni.harvard.edu>
Co-authored-by: Oscar Smith <oscardssmith@gmail.com>
