[llvm] Edit Semantics Description of `@llvm.is.constant.*` (PR #77519)

[Joerg Sonnenberger via llvm-commits]

jsonn wrote:

The Lower-Constant-Intrinsic pass will remove all instructions that haven't been folded otherwise. It folds to true for material constants and false otherwise. As such, the statement is false for all normal optimization pipelines. 

https://github.com/llvm/llvm-project/pull/77519