[llvm] [TargetLowering] Add a new function `getNullPtrValue` (PR #126665)

[Shilei Tian via llvm-commits]

shiltian wrote:

I have not updated other test cases yet. Just to make sure this is the right thing to do, or I didn't miss any historic reason here. The GISel will be a follow-up (if necessary).

https://github.com/llvm/llvm-project/pull/126665