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

[Alexander Richardson via cfe-commits]

arichardson 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.

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