This is needed to support the new interleave intrinsics from D141924 for
fixed vectors.
I've reworked the core loop to operate in terms of half of a source. Making 4
possible half sources. The first element of the half is used to indicate which
source using the same numbering as the shuffle where the second source elements
are numbered after the first source.
I've added restrictions to only match the first half of two vectors or the
first and second half of a single vector. This was done to prevent regressions
on the cases we have coverage for. I saw cases where generic DAG combine split
a single interleave into 2 smaller interleaves a concat. We can revisit in the
future.
I don't understand this if condition. It looks like you're leaving EvenV unset for the low element of the vector. That probably can't be right (as your tests cover that case), but there's something I'm missing here.
Ah, I think I see what's going on here. You're trying to handle undef vectors (i.e. we never matched the other polarity). I think this is dead code in the current patch, and these should be probably be asserts.