[llvm] [InstCombine] Extend bitmask mul combine to handle independent operands (PR #142503)

[Jeffrey Byrnes via llvm-commits]

jrbyrnes wrote:

> Thus, to be sufficient for my usecase, it seems that reassociation must be able to handle the case where one or more of the leaf forms are and->icmp->select (instead of and->mul)

I see that I did not make this explicitly clear in the description, nor did I cover it in any tests. Sorry about that.

https://github.com/llvm/llvm-project/pull/142503