SE: Learn NotNull from ??= for nullable types

[Tim-Pohlmann]

arg ??= 0;
if(arg is null)
{
  // This should be unreachable
}

??= is being translated to a check of .HasValue. Currently, the engine cannot detect the above case, because it does not learn NotNull on the arg.HasValue branch.
See original discussion:
#7312 (comment)