[InstCombine] Missed optimization for select a%2==0, (a/2*2)*(a/2*2), 0

[XChy]

When I played with InstCombine, I generated something missed-optimized.
No real-world motivation, but it seems to be easy to implement.
Missed example.
Proof.

[jf-botto]

I'd love to work on this if possible?

[XChy]

I'd love to work on this if possible?

Sure, thanks for contribution.

[nikic]

Note that the mul here isn't really relevant. Here is a simpler pattern that is also missed: https://alive2.llvm.org/ce/z/dYwE-Q

The general issue is making use of KnownBits information inside select arms.

[jf-botto]

Thank you @nikic,
So what you're saying is, the fix for this is optimising the RHS/LHS of a select by taking into consideration any KnownBits that can be inferred from the condition of the select?

[jf-botto]

Making good progress on this, just have some edge cases that need tidying up.