[clang] [InstCombine] Add combines/simplifications for `llvm.ptrmask` (PR #67166)

[via cfe-commits]

goldsteinn wrote:

> Since the langref definition is based on GEP, can we use index width instead of pointer width? If you disagree with this, it would still be good if you could add a test where the datalayout has index width < pointer width to show the current semantics.

I think thats fine although am not sure of the proper subtlies involved.
@nikic does that sound reasonable?

https://github.com/llvm/llvm-project/pull/67166