[GlobalISel] Handle IntToPtr constants in dbg.value

Currently, the IR to MIR translator can only handle two kinds of constant
inputs to dbg.values intrinsics: constant integers and constant floats. In
particular, it cannot handle pointers created from IntToPtr ConstantExpression
objects.

This patch addresses the limitation above by replacing the IntToPtr with
its input integer prior to converting the dbg.value input.

Patch by Felipe Piovezan!

Differential Revision: https://reviews.llvm.org/D130642