[GlobalISel] Handle nullptr constants in dbg.value

Currently, the LLVM IR -> MIR translator fails to translate dbg.values
whose first argument is a null pointer. However, in other portions of
the code, such pointers are always lowered to the constant zero, for
example see IRTranslator::Translate(Constant, Register).

This patch addresses the limitation by following the same approach of
lowering null pointers to zero.

A prior test was checking that null pointers were always lowered to
$noreg; this test is changed to check for zero, and the previous
behavior is now checked by introducing a dbg.value whose first argument
is the address of a global variable.

Differential Revision: https://reviews.llvm.org/D130721