[llvm] [ExpandReductions] Add support for non-pow2 vector counts (PR #208738)

[Simon Pilgrim via llvm-commits]

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RKSimon wrote:

canonically InstCombine expands vXi1 or reductions to:
```
%1.bc = bitcast <3 x i1> %1 to i3
%2 = icmp ne i3 %1.bc, 0
```
(and reductions to eq -1)
either we update the tests to do the same or handle this in ExpandReductions 

https://github.com/llvm/llvm-project/pull/208738