[SelectionDAG] Handle IntToPtr constants in dbg.value

The function `handleDebugValue` has custom logic to handle certain kinds
constants, namely integers, floats and null pointers. However, it does
not handle constant pointers created from IntToPtr ConstantExpressions.
This patch addresses the issue by replacing the Constant with its
integer operand.

A similar bug was addressed for GlobalISel in D130642.

Reviewed By: aprantl, #debug-info

Differential Revision: https://reviews.llvm.org/D130908