[x86] try harder to form 256-bit unpck*

This is another part of a problem noted in PR42024:
https://bugs.llvm.org/show_bug.cgi?id=42024

The AVX2 code may use awkward 256-bit shuffles vs. the AVX code that gets split
into the expected 128-bit unpack instructions. We have to be selective in
matching the types where we try to do this though. Otherwise, we can end up
with more instructions (in the case of v8x32/v4x64).

Differential Revision: https://reviews.llvm.org/D72575