[DebugInfo] Don't drop dbg.value's of nullptr
Currently, dbg.value's of "nullptr" are dropped when entering a SelectionDAG --
apparently just because of an oversight when recognising Values that are
constant (see PR39787). This patch adds ConstantPointerNull to the list of
constants that can be turned into DBG_VALUEs.
The matter of what bit-value a null pointer constant in LLVM has was raised
in this mailing list thread:
Where it transpires LLVM relies on (IR) null pointers being zero valued,
thus I've baked this assumption into the patch.
Differential Revision: https://reviews.llvm.org/D55227