[llvm] [InstCombine] Support multi-use values in cast elimination transforms (PR #165877)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> @dtcxzyw should we ping another reviewer directly? Did you have someone concrete in mind?

Given that there have been no further comments in the past two weeks, I think it is ok to merge this patch w/ test fixed. Nikic is currently out of the office, and he will come back in Dec.

https://github.com/llvm/llvm-project/pull/165877