[llvm] [SystemZ, ArgExt verification] Cache results of isFullyInternal(). (PR #130693)

[Ulrich Weigand via llvm-commits]

uweigand wrote:

In the end, the information we need is the same that is already computed by `Function::hasAddressTaken`.    A function that does not have its address taken and has local linkage is fully internal to the current module - this same check is already being performed by other optimization passes.

So I think in the long run it would be best for the SystemZ back-end to simply use `hasAddressTaken`, and then -if necessary- optimize that common code routine by adding a cache.

https://github.com/llvm/llvm-project/pull/130693