We can convert a shift-left-by-constant into a multiply (we canonicalize IR in the other direction because that's generally better of course). This allows us to remove the shuffle as we do in the regular opcodes-are-the-same cases.
This requires a small hack to make sure we don't mistakenly introduce any extra poison:
The other examples of opcodes where this would work are add+sub and fadd+fsub, but we already canonicalize those subs into adds, so there's nothing to do for those cases AFAICT. Are there other opcode pairs where we can do this kind of transform?
Note that there's a different fold needed if we've already managed to simplify away a binop as seen in the test based on PR37806, but we manage to get that one case here because the fold is positioned above the demanded elements fold currently.