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

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NCGThompson 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.

There is an exception with `alwaysinline` if `@llvm.is.constant` is called directly on a function's input. `@llvm.is.constant` while be optimized to `true`, but the `true` won't be propagated further with -O0.

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