[x86] improve codegen for bit-masked vector compare and select (PR46531)

We canonicalize patterns like:
  %s = lshr i32 %a0, 1
  %t = trunc i32 %s to i1

to:
  %a = and i32 %a0, 2
  %c = icmp ne i32 %a, 0

...in IR, but the bit-shifting original sequence may be better for x86 vector codegen.

I tried several variants of the transform, and it's tricky to not induce regressions.
In particular, I did not find a way to cleanly handle non-splat constants, so I've left
that as a TODO item here (currently negative tests for those are included). AVX512
resulted in some diffs, but didn't look meaningful, so I left that out too. Some of
the 256-bit AVX1 diffs are questionable, but close enough that they are probably
insignificant.

Differential Revision: https://reviews.llvm.org/D83073.

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