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

[via cfe-commits]

goldsteinn wrote:

> We should update LangRef and require that the integer arg has the size of the pointer index type 
As in disallow say `ptrmask.p0.i32` on typical systems with 64-bit pointer/index?

> (with current GEP semantics implying a 1-extend to pointer type width), implement a Verifier check for that, and add an assert in SDAG lowering that the index type size is the pointer size -- I don't believe we currently have any in-tree SDAG support for pointers with narrow index types, so this is just a breadcrumb for out-of-tree targets to implement a 1-extend at that point.
> 
> I was planning to do this, but never found the time.



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