[llvm] Add isa_or_null<> to Casting.h (PR #104682)

[Craig Topper via llvm-commits]

topperc wrote:

I'm confused by #50305. It compares `isa_or_null` to `dyn_cast_or_null` and `cast_or_null`. Both return null if the pointer is already null. The proposed isa_or_null returns true if the pointer is null making it different.

How many places are there that want true for null?

https://github.com/llvm/llvm-project/pull/104682