[llvm] [GVN] Look through select/phi when determining underlying object (PR #99509)

[via llvm-commits]

goldsteinn wrote:

> getUnderlyingObjectThroughPhisAndSelects() (let me know if you have a better name...) 
maybe `getUnderlyingObjectAggressive` to leave more room for future extensions :)

https://github.com/llvm/llvm-project/pull/99509