Use pmaddubsw for non-RDom horizontal widening adds #7440
Merged
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We were rewriting the following into a pmaddubsw:
But not the equivalent unrolled version, which is more likely to be written:
This pattern shows up in downsampling code.
This PR fixes it, and also adds support for doing horizontal widening adds of i16 to i32 using pmaddwd, and adds some missing avx512 variants of horizontal adds using pmaddubsw.