[llvm] [InstCombine] Simplify `floor`/`ceil` to `trunc` for integral tests (PR #190620)

[via llvm-commits]

dzaima wrote:

Note that this results replacing one round function with two round functions if the `floor`/`ceil` is also used for something else - https://c.godbolt.org/z/795MKfhva (not something from any code I've written, but seems quite likely that it's done by some things)

https://github.com/llvm/llvm-project/pull/190620