Pure null tests should affect both branches

[jcouv]

From LDM 2/20/2019 and 3/6/2019 (adjusted {} and derived is Base):

"Pure" tests should set the null-state of x in both branches:

1. x == null
2. x != null
3. (Type)x == null
4. (Type)x != null
5. x is null
6. x?.F
7. x?[i]
8. x ?? y
9. bool [EqualsBehavior]MyEquals(object other)

Note: pure tests 1 through 5 (included) are implemented or verified in #33929

"Not pure" tests should only set the null-state of x in one branch:

1. x is Y // where Y is not a base of X
2. x is Y y
3. x is Y _
4. x is C { Property = 3 }
5. TryGetValue([NotNullWhenTrue])
6. [NotNulllWhenFalse] string.IsNullOrEmpty(s)
7. derived is Base
8. x is { }

switch expressions and statements should also use this notion of "pure test" (LDM 3/6/2019).

When closing this issue, we should also close #33526 (and remove references to both issues from code)