[NVPTX] instcombine known pointer AS checks. (#112964)
The change improves the code in general and, as a side effect, avoids crashing
on an impossible address space casts guarded by `__isGlobal/__isShared`, which
partially fixes https://github.com/llvm/llvm-project/issues/112760
It's still possible to trigger the issue by using explicit AS casts w/o
AS checks, but LLVM should no longer crash on valid code.
